Maybe there's already something like this (thought I would ask before trying to make it):
When Admin bans a user, he has a custom field for a URL that the ban-ee will be redirected to. Customizable on a per user basis.
Examples
Send USER #1001 to a thread
about banning
Send USER #2234 to a
self-help group
Send USER #3292 to
End of the Internet
Send USER #4371 to the "You are an idiot" page
And so on
Regards,
Matt