I have not seen this suggestion before, but if I missed it .. sorry in advance.
SUGGESTION: Place some BROAD rules on pm "etiquette", allow some kind of recourse for very SERIOUS violations. Something like first time a warning, second time removeal of pm privilages (for a time at least) and finally (if pm reinstated and still happens) suspention from site.
SPECIFICS: For very SERIOUS issue ONLY, either allow folks to take some kind of recourse against those who send virulant pm's (threats, huge volumes of email, continuing after requests to stop), perhaps somehow allow folks to filter their pm's for severe language. IF neither of those is practical, perhaps there could be a way someone can, in the most serious cases ONLY, ask admin to block a particular person from pm (either totally or to a specific person).
WHY NEEDED? I know folks have suggested various filters and so forth. Mostly, I have felt that that is more trouble than it is worth. If you don't like someone' language, just put them on your ignore list and be done. EXCEPT that doesn't stop pms. On the one hand, I am an adult and can quite easily hit the "delete message" "button". On the other, why should I, or anyone else have to put up with virulant attacks just because we had the misfortune to be in a game with a real jerk.
IF a rule is implemented would have to be VERY broad. For example, a flat rule stating no abusive language would NOT be appropriate. HOWEVER, a rule forbidding threats specifically, a rule against huge amounts of pm, AND/OR a rule stating that if someone ASKS you to stop pms, considers them harrassing, there is admin recourse if they don't .. that might be appropriate.
Again, I am not talking about the run of the mill, one time, "you are a jerk" email. I am talking more serious stuff -- real threats, real virulance, large volumes that clog the inbox .... particularly messages that continue AFTER requests to stop
