Is each forum, set to registered?
Or you have it set for everyone?
I had a problem where someone came onto my forums and posted a bunch of poker site posts. Found out, that my forums were all the way open so, guests can post.
So, I went in and changed each one to where you have to be registered to post.
If you don't see them in the member list, then that is probbaly how you have it set up to where guests can post.