I've had the same problem on 20+ boards. Things to check for....
They like to use usernames starting with "a" or zero, so that they appear on page one of the user list.
Check the userlist, particularly those on the first couple of pages periodically for user's with cialis, viagra, diet pills sites as their home page.
Turn off the user list for anonymous users.
I collected a bunch of information on the "000" user that was posting everywhere. Kept denying thier membership with the reason: "Spammer: IP address logged for legal action."