i have been running forums for 10 years now, with max 500k uniques daily and 5k threads per day. i think i know the people you are talking about, and the one and only thing is "do not feed the troll". he was there to make problems, he will try to make problems in future, and if you want to make problems you need the second side. just stop communicating with him and he will end in nervous agony one day
believe me, that is the only one way, there is no other!