A suggestion to go along with all the others: keep your interaction with the offender to an absolute minimum. The quickest way to discourage activity like this is to make your group a black hole to the offenders. I suggest keeping new members moderated. When you identify a repeat offender who signs up under new names, don't reply, just shut them off. They join, post offensively, and nothing happens-- no reply, the post is not distributed to the group. The only response is silence. This is the most discouraging situation imaginable to a troll.They quickly move on to someplace that gives them the angry response they crave.
Best, Marv
Marvin Waschke