I agree that many posts made recently by the new members don't contribute so much to the threads. However you should go easy on the new members.. This thread should be enough to inform new members about posting, I don't see the need of PM:ing all of them or whatever you are about to do.
You do realise that this "10 post to get in the beta" was planned by the developers to hit two birds with one stone, not only do they get beta testers but also do they get the forum activated with fresh members and more posting.
Don't scare them away while they are new and easely turned off by the forum