Suggestions needed, please.

I have an annoying twit who keeps sending me messages telling me that he
wants to get off my list. As far as I can tell he doesn't have a
subscription. He claims he only has the one email address. I checked for
permuations of the domain in my subscriber list and nothing. The routing
information on his email uses the same domain. My list is not gatewayed or
otherwise redistributed as far as I know. I checked the logs from my smtp
server, too, and could find no reference to this person other than mail I
sent him directly.

I asked him to forward me a copy of a message he received from the list,
but I haven't gotten that yet. I know that headers from that should tell me
something, but while I wait for that, does anyone have any suggestions on
how to track this person down and delete him?

Thanks for the help,
Karen Strauss