You don't even have to do that. Just assert copyright in the "Welcome to
the List" message, and (if you want to be legally sound, more or less)
requiring the new subscriber to reply with the contents of an agreement
included in the Welcome, agreeing. If not, no subscribe. 'Course, you'd
have to at least try to get existing members to do this too.
>You could also cook up a scheme where each list member signs over copyright
>of their message to you, then you include a copyright notice with each
>message. Then you can attempt to have the annoying person's admin shut
>them down for copyright violation. This is stretching things pretty thin,
>though (and, of course, IANAL).