Why not implement something like this:
For newer members: "Your message have been submit successful. For the comment to be viewable by others, one of the administrator will have to approve your message." (if one of the administrator don't have the time to approve the message, the post will not appear on the forum until further notice from the administrator); for the existing members the message posting will work further.
From my point of view in this case the spammers will be less or maybe the threads will not appear in the future.
What do you think?