Yeah, it's a real problem - on vbulletin's I run I installed a script on each to automatically disallow members with less than 15 posts from PM'ing or posting a link.
Seems to work, especially against those "Free ipod!" thread starters.
It's a pity that you can't have an automatic anti Bo**ocks detector during the pre 30 post, 30 days period.
There is a great difference between posting "I agree with that" or similar and posting a really useful reply that adds value to the forum.
We do try and delete posts with no real value - especially those who posts multiple uselessness in order to increase their post count (we know who you are ) BUT it is a fine line between deleting useless posts and laying down the law to an extent that nobody comes back!