After deleting like 3 or 4 spam guest posts, I've changed the permissions of all forums: now, guests are only allowed to read, but not to write. I left the "New members" forum opened for guests, but this morning I found a "nice" job offering from a guest in this section, so...
This IP is banned and the guest posting, disabled
![:ninja:](http://ntsms.megatherion.com/Smileys/default/ph34r.gif)