It needs a bit explanation, i.e. what do you mean by "until they are approved". Where the user is approved and where it's not, in your point?
I meant until users (new users) are approved through premod addon.
IMO fighting with PM feature abuse is like fighting with your own shadow :). There's a "Report" link in PM so if someone finds the message offensive, s/he can always report it. Even if there's a restriction that a member could send a PM only after posting X messages, it doesn't help at all - because you end up with 10 useless messages on the public forum and the PM is still abused.
Yes true there's a report link in PM but till the user has been reported, he/she might have sent tens of PMs.
The Premod addon does an excellent work of pre-approving new users. However, unapproved users can still send PMs.
People who are here to spam, simply don't have time to post anything worthwhile. They simply post spam and vanish. It's almost like a formula and it works the same way most of the time.
I know you can't prevent abuse altogether but by preventing new users from making PMs until they are approved (through Premod addon), you can at least prevent outright abuse.