Are you sure that these spams are coming from registered accounts and not guests?
I had the same problem and I really thought that spamers were registered users (the confusion is possible because in other forums, only registered users have a link on their name).
in fact, my porblem was coming from the fact that my punbb forum has been created by converting a phpbb one.
In that case, in the table users, the guest group_id was equal to 4 instead of 3.
Because of this little mistake, guest posting is active even if you have set your permissions to authorize members posting only.