IMO the people who threaten violence are much worse than him. I would consider posts inciting/advocating violence or harrassment to be a clear breach of the community guidelines here.

The guy's weird and has a weird fettish, but tricking people to have them sit on you is something that requires counselling, I don't know that it would even fit the definition of an assault. Maybe there needs to be some sort of social intervention order that can be used for people who don't target a single victim, that can tell someone their actions are unacceptable and if they can't follow it to make it possible to put some sorts of interventions in place.