Hi David,
we had a registered user which turned out to be a SPAM engine (now we know).
we deleted the user account - but SPAMing continued, now the user name was "unknown".
in this case neither deleting the user nor banning the IP was of any effect.
then we realized that the session was still running. we killed the session and this was the end of the SPAMing - so far...

Best wishes,
PSPGadmin