Personal Messages Whitelist of users

[Court]

This has probably been requested before.
Looking for a way to prevent an unknown user from spamming personal message areas or harassing users.

Any help much appreciated, even if its an explanation as to why its really very difficult to do.

Thanks for your consideration.

[scottalanmiller]

I think that the hardest part is figuring out how to define "known users" without making it a big manual hassle that essentially breaks the usage.