I feel blessed that anyone can show up here and send pm's right away, but that it takes 50 posts to rep.
I'm pretty sure you can set it so people can't send PMs until they reach a certain number of posts. TrekBBS does it, so it can't be that hard.
I set the Newbies and Users Awaiting E-Mail Confirmation usergroups to not be allowed to PM...I think those are the only two new spammers should be falling under. So let's see if it works.