Blocking Private Messages

[ashaard]

Hi everybody!
I've got a big problem with my SMF forum and I hope, than you can help me.

Well, there are some guys who makes new accounts and sends private messages to users (this PM's include porn and advertise links).

So, the main question is: how to block sending private messages by new users? For example, for users who wrote less than 20 posts, or something like that. What should I do to block it?

Sorry for my weak English but I hope you understand my problem .

[CapadY]

Make post dependent membergroups.
No PB's for membergroups under 20 posts.