Most people would say the way to deal with a Dementor is with a Patronus, but there's a bit of evidence to the contrary.

First there's a statement from Harry in Chapter 21 of HBP:

For there to be a best way to tackle Dementors, there must be more than one. The method Harry knows is of course the Patronus, and this other method(s) must be legitimate seeing as it's coming from Snape, who is rather good at the Dark Arts and defending against them.

Speaking of the Dark Arts, Dark wizards are stated (by WoG, I believe) to be unable to cast the Patronus Charm. Yet Voldemort and other Dark wizards throughout history are able to control them, and control must inevitably include the ability to decisively deal with the controlled if they ever get uppity. In addition I find it difficult to believe that a witch of Bellatrix' calibre for instance would be helpless in the face of a single Dementor.

For an explicitly canon option, we see in DH that shades of the dead summoned with the Resurrection Stone can act as protection against the effects of Dementors. Which brings up a whole lot of interesting questions with respect to how the Stone works, but doesn't really contribute much to the issue at hand; that can hardly be the method Snape recommended.

So what could it be? We know Dementors are amortal, but how does that affect how magic interacts with them? There is no indication that you cannot for instance conjure a box or other trap around a Dementor; they're not noted to have super strength or to be able to pass through walls, so in theory it should be unable to escape. There's a flaw in that plan, though - its effects would likely still be felt through the box. So what happens if you take things a step further and Vanish the whole thing? Or if you take the tried and true method of setting it on Fiendfyre?

Of course, it should be noted that the presence of a Dementor would severely shake the focus of most wizarding folk, but the exceptional ones should be able to cast spells under duress.

Thoughts?

 

I think you've made a few assumptions here.

For example, with Harry's disagreement with Snape, there are other explanations too. For example, they could be disagreeing over the best way to cast the Patronus Charm (we know that Harry, as of OotP, can cast it in a slightly different way to the standard procedure, using the thought of his friends rather than a specific memory). Alternatively their disagreement could be over strategy rather than magic: Snape may consider discretion to be the better part of valour and advocate avoiding them in the first place (or running away), whereas Harry advocates tackling them head on.

Another assumption is that it is shades of the dead in general that protect from Dementors. It seems equally likely, if not more so, that those specific shades protected Harry due to his specific relationships with them and the role that their deaths play in Harry's worst memories, which the Dementors usually exploit.

With respect to Dark wizards and the Patronus, JKR didn't say Dark wizards cannot cast the Charm (indeed, we know that Snape can at the least). Rather she said Dark wizards do not need to cast it. This goes well with what Dumbledore said in GoF about Voldemort being the most natural ally for Dementors and other Dark creatures. There's a vague but very real way in canon that Dark wizards are more than just wizards who happen to cast Dark magic - there's something about them as wizards that is different too (this fits nicely with my idea of a person's character/being shaping their magical identity including power(s), dispositions, intuition, etc).

I feel like we already have a fairly good position in Dementors:

1. They are invulnerable to direct attack, physical or magical.

2. They can be warded off with the Patronus Charm.

3. While you cannot "kill" individual Dementors, the body of Dementors as a whole is like a single mega-organism which grows and shrinks (thus increasing or decreasing their numbers) depending on the condition of their environment, thriving in places of decay and misery and presumably therefore being driven back by places of happiness and renewal.

 

I considered the former, but neither really mesh with the syntax of the statement in my opinion. But let's say that's true.

That might answer the why, but not the how. Unless you're arguing that simply being around [living] people who are connected to your worst memories or simply being around people you love protects you from Dementors, there is something about them as shades that enables them to actually do the protecting.

Actually she (later? I'm not sure of the timeline) said that Dark wizards who try to cast it (may) get eaten by maggots, and calls the belief that they cannot cast it justified.

And again I think this is answering the why but not the how. Sure, Dark wizards are Dark creatures and natural allies of other Dark creatures, including Dementors. That still does not tell us the means by which they can assert the essential control over their would-be allies that they need to, especially considering that simply being Dark is no protection from a Dementor's effects: they fucked Death Eaters and other criminals over too.

1 is what we're currently considering (also, physical? I'm not aware of anything preventing them from being physically interacted with). 2 is obviously true. 3 is...not directly supported by canon, as far as I'm aware.

 

This seems like an odd thing to take away from your link, since it explicitly says that Dark wizards can cast the Patronus but often find it unnecessary. The key, as the text notes, is "true and confident belief in the rightness of one’s actions". Which actually describes most evil people.

Three is word of god, though also supported by the Pottermore entry on Azkaban.

I'm not sure what there is to consider on the topic of #1, as both Remus Lupin and the Pottermore article you link state that the Patronus Charm is the only effective spell against Dementors.

 

I would agree that perhaps Umbridge is unaware of her sadism or personality faults and feels she is doing her duty in crushing rebellion from what she sees as a perfect society.

However, Bellatrix is equally self-righteous. She is a more skilled witch of a more explosive temper (Umbridge's sadism and usually quite calm in comparison) but she genuinely believes a society of pure-bloods is stronger and that muggle-borns are unworthy of magic and life.

Dementors have no effect on her at all, it seems. She shows none during her trial whereas Crouch is in tears, screaming and shaking at their effect and we see they also affect Karkaroff too, although not to the extreme degree they effect Crouch. This would suggest that Bellatrix is even more self-righteous in her ideas than Umbridge and the Dementors have nothing to feed on - like, they could not feed on Sirius as he held on to the fact that he was innocent which is not a happy or sad thought but a statement of fact in his mind.

Perhaps it depends on the selflessness of the memory and Umbridge did have one, deep down, whereas Voldemort and Bella would have no ability to use a Patronus as their happiest memories are likely or murder, torture and causing untold suffering to others.

Then, what of Lucius though? He, for all his faults, truly cared for his wife and son and would likely have happy memories locked into them.

It is a conundrum, actually.

 

I had an idea that werewolves could be a threat to dementors. Their wolf natures could be similar to how Sirius hid in his animagus form. Also, the hate and rage of the change, could counter the emotional effect near dementors? The teeth and claws of the werewolf are cursed, are they powerful enough to cause damage to a dementor?

 

While the PC might be the only spell to directly effect them, I don't see why you wouldn't be able to affect them indirectly. Dementor's aren't ghosts - they can't go through solid barriers. Like, the Dementor's couldn't get at the prisoners behind bars. So, I don't see why you wouldn't be able to control their movement by moving barriers around. It's not like it's the unstoppable force meeting the unmovable object.

 

According to Dumbledore Voldemort can offer the dementors a greater scope for their powers. Which I always took to mean that the Voldemort would offer more victems. Perhaps we are thinking too much about this, and perhaps dark wizards just offer someone else in exchange for their own survival.