The common situation about such case is to let them know that there
 is a reason behind having thousand of lists with different titles, so
 people can discuss about related material in the particular lists.
 The first amendment does not apply if a stranger wants to enter your own
 house without your permission. Tell them to get lost if they are
 disturbing the list! If they don't like the way you run the list they can
 create another if they have the means to do so.
 Put the list under moderation(temporary) and let others know that the
 moderation will effect ONLY the trouble makers.
 I had to do these on one of my lists and everything is OK now...

later,
Mentor