private messages approval system ?
 

is there a plugins or product / hack that can do pm moderating ?

so before sending it directly to other users admins should check if there are any spams of invitation to other forums ..

What you could do is disable pms for one group, allow users to post around a little bit (moderate their posts for like a week) then have an auto promotion after x amount of time, and y amount of posts. (where they can then PM and other things you'd like them to do.)

is not that i didn't do that , but the problem with mods and special users
i have security issue with some moderators sending spam to vip and special users , i wont relieve them from their authority just for speculation

so i believe " pm tapping/moderation " is the best solution

i think if there are report pm it will be nice addition as default of vbulletin

reporting pm is available in v3.8b

the problem not so many users have the courage or willing to report mods about bad behavior or rules violation