This is the first I've heard of people blaming accessibility tools for anything.
You gave the example of alt text being used to hide slurs, but I'd be pretty confident this would be handled the same way as if they used them in the open. It would be brought to an admin or mods attention, they would confirm, and take appropriate action. Trolls regularly use slurs in the open so I don't see why this situation would be handled differently.
Are there other examples of accessibility tools being used for bad things?