Are you sure they are trying to relay or bounce mail from your server? Based on your commentsbelow about messages to: people and the passing of the relay tests it appears this might be anattempt to flood your mailserver with pseudo-random names trying to actually find validusers to send spam to or collate a list of valid usernames at your domain. Either way I wouldwant them blocked.
> How can I be sure I'm not relaying for this scumball? How can I just > block him completely? > > Are there any good test tools out there for making sure I'm not letting > any scum get through? >
