Are you under any contractural obligation to transit that IP
address? The user in question seems to think you are, but you
should check that as well; most contracts that I've seen do
not mention multihoming specificially, and this could be the
perfect loophole for you to use while you give him the 30 days
notice or whatever it takes to disconnect him completely.
Lawyers have a funny way of reading contracts. Some read them that anything
not spelled out is allowed, whereas others read them that anything not
spelled is not allowed. I guess ISPs on NANOG should be aware of this new
spam/ip-spoofing method when writing contracts with their clients.
-Hank