I don't know if that seems like a good idea to you but I could easily imagine a ratio for everyone and the day you drop below a certain point you can no longer post on the forum.
I don't like the ide to prevent an user for posting. First because he can create a new account and spam again, and also because of the censorship philosophy which I don't like.
A guy may never been merited and is a complete shitposter like pethesteman friends... You can just ignore him.
However, I believe that signatures should be disabled for users which never earned at least a few merits.
You don't need to ban shitposters from posting, but they should never receive in incentive to shitpost.