If your list is one where only members can read postings, and only approved requests can add a member, then kicking off an abusive private emailer might be effective. If, on the other hand, there are publicly readable archives, or anyone can join, then it is hard to prevent a recurrence of the problem UNLESS member addresses are "cloaked" in the archives, postings, or both. (Cloaking member addresses in publicly readable archives is also a good anti-spam measure.) You might also keep a list of mail-filtering resources handy, and forward it to any complaining member. Teaching them to protect themselves from harrassment will have benefits that outlast the initial crisis.
