Well, just for the record, you can limit PMs to the people on your friends/contacts list and staff only, Brother. If you enable that in your settings, then people who are not on your friends/contacts list cannot send you PMs, unless they are staff members. But of course, then it comes down to your own moderation beforehand regarding whom you want to add to or keep on your friends list, and whom you will be denying the privilege.
In the end, it only moves the conundrum to a different aspect of your forum interaction, but it might be easier to mitigate this way.