One alternative (if the ACL feature in the software allows it) is to implement a Newly Registered Users list such that when one registers on TM one is also added to the new users list, which has a post count parameter. When the user reaches the post count (Say 50 posts or whatever) then they would automatically be deleted from the new users list.
I have looked at this before but was concerned that it promotes speed-posting of rubbish. I have seen it time and time again on other forums.