AFAIK, all the people who have been banned share one characteristic, which was evident long before they were banned, and pretty much identified them as a likely future "bannee".
When they got involved in threads, the threads frequently deteriorated into insults and name-calling.
Yes, it takes two to squabble, but it was always notable that the "bannees" were always one of the two. The confrontational, goading, needling way they interact either starts an insult-fest, or provokes others to start one.
Its not that subtle, really - a troll is a troll.
Mule goes through phases of being more troll-like and less troll-like. He's currently in a slightly less troll-like period, in my opinion. But he's still my candidate as the most likely to eventually go over the edge and get banned.