Just had the following play on our server:
http://steamcommunity.com/id/suckmygoddamnpenis/
For some reason, we were not able to ban him, and he was using the name " SourceMod Anit-Cheat bypass".
I was able to go and ban him manually via web, but this was really strange.
EDIT: I should also note that his Steam ID was being repoted by the system as:
"STEAM_ID_STOP_IGNORING_RETVALS"