I decided to see if I could fix this. Haven't done much with the admin panel yet, and my access is limited, but I have been able to tweak the New User group permissions in order to prevent them from PMing with no approved posts. It's been tested and confirmed to work, and once approved, PMs become open to them, so we hopefully won't get any new users PMing crap anymore. We may have some sleeper accounts that could still send them, but this should hopefully limit them too.
Any long term users who suddenly find their PMs are no longer working, please post in the General Forum, and I'll make sure you're added to the correct user group.
Hopefully that's the last we'll see of PM Spam.