On the lists at USC, I give the listowner final say in what is appropriate.
I run listserv, not any list in particular, and cooperate with the listowner.
Generally, we do what is necessary, including (not necessarily in this
order):
 
1.  ask nicely to cease and desist non-topical discussions.
2.  unsubscribe the person
3.  serve the userid off (so he cannot resubscribe).
4.  use smtp to intercept all mail from the person
5.  notify the person's University or organization.
 
I have invoked 4 & 5 only once so far, and the University where the
offender was a student shut down his account and took disciplinary
action against him.  If a person resists #1, document all the correspondence
in case you get to #5.
 
jr
(john riehl)