The time limit I find useful.. The number of posts not so much. You'll have a bunch of people posting "oh yeah" and "hi" messages into other threads to hit that amount. Anyone going out to troll will say "Oh boy! A message board I can totally go 4chan on!" then get a message like "You must wait 4 days to post images. Please enjoy the other aspects of our message board until the required time has passed."
They'd probably go elsewhere to troll and not bother holding onto the link to even come back. And if they do.. well, they're quickly banned and their mess is mopped up.