Every list has a purpose, and if you are fair in holding subscribers
 to that purpose, then I can't see how anyone could fault you.  In short
 if the intent of the list precludes the posting of politcal views or
 ending mail with "penis & vagina signatures", then the poster is out of
 line.  Things get more complicated if there aren't well defined rules
 governing the use of the list of course.  As someone else already added
 "welcome to listownership!".  There are times when you are bound to
 step on someones toes no matter what course of action you choose.
 
 I have a list where the topics for discussion are loosely defined as
 "whatever the people on the list think is appropriate", and something
 similiar came up a while ago.  Given the perversely democratic nature
 of the list, I simply asked the list members to send their opinions to
 the person with the suspect signature file, and asked that person to
 abide by the prevailing opinion.  He stopped using the controvertial
 language soon after that. :)  You might want find it helpful to ask
 real people to *politely* express their opinions to the poster.  Since
 they aren't evil list administrators they may be taken more seriously.
 
 -jj