Hi,
This has happened to me on 2 forums, the Kubuntu forums, and the Comodo forums. Somebody sends a lot of PM’s to you containing porn and posts a lot of topics in all the boards containing the same information in the PM, please be on the alert Alwil and Forum Users.
You can set people on ignore for msg’s in your board prefence settings.
Thats usually a problem with forums which allow Guest accounts, though regular ones aren’t immune either.
I also got that crap in my PM inbox (4 times!) so i’ve already sent that message to Melih requesting hi to kick that user and ban his IP.
EDIT:
Just checked and Melih already dealt with the issue 
just like on the avast! forums you must be registered, it only showed him as guest because melih kicked him.