The troll filter
Many years ago (94-95) I was running a community but as all communities we had our shares of trolls in there. I tried to ban them, delete their account and all I could imagine, but of course they came back with a new user.
I grew extremely bored of them and changed tactic, I added functionality and made them invisible. They could se everything, comment on everything and participate in all activity, but noone ever saw their content, it just showed up for the trolls. All trolls could se each other and respond to each other, and sometimes the system added a comment on them with something like "a**hole!".
My users were happy, and the trolls kept busy trolling each other.