Rat him out to the volunteer coordinators of the spaces he’s trying to join. Anonymously, if you want to. Say that you’ve noticed one of their volunteers looks like a criminal who goes by several aliases with a history of violence in fandom spaces, and send them to his arrest record or some other record of his legal history. Don’t minimize; actually use the phrase “aggravated assault with a deadly weapon” or whatever the charge was that led to the arrest. Tell them point blank that you are concerned for the safety of his fellow volunteers and, even though tattling on people’s substance abuse problems sucks, tell them that you are concerned that his illegal drug use will put the entire convention at risk. If he’s using a fake name, it shouldn’t be difficult for the people running the convention to ask their volunteers for a valid drivers license or other state ID for liability purposes.

This guy sounds like a much more clear-cut danger then Andrew Blake, honestly — Andy is a chronic abuser and manipulator, including manipulatinf violent people like Brittany Quinn’s ex and potentially violent people like Chris C. But he doesn’t seem to be a risk for committing immediate physical violence against anyone— if this guy tried to stab someone over a phone bill, he’s an immediate danger and the people supervising him need to know.