I run a list which is really a course in fiction writing with a heavy
volume of mail, 99% of which should be writing submissions and critiques.
I just threatened a chatty newcomer with the REVIEW command.  Haven't ever
used it, but the manual says you can set someone to review and all their
messages will come to you for approval.  That ought to get everyone's
attention if they are seriously off-topic and polite requests have failed
(as they did for me).
 
Jilla