We rely on the community to tells us about these creeps. PMs are often abused by trolls like the one mentioned above, those seeking BBFS, digging for personal info, the list is endless.
If you get a PM you take exception to, you can and should flag it just like a post, using the Report button. The mods will dig into it and take the appropriate action. Don't just delete it and expect someone else with deal with it, do your part to keep PERB a good place to share info. Ladies, this goes for you as well, report the guys who are trolling you or pushing your boundaries, don't just suffer in silence.