Unlike torrent websites where you can make a ratio because you can create a torrent and seed, in this case, you won't be able to earn merits if you can't post.
So once you reach the quota/ratio whatsoever basically it's one false move and it's goodbye for that account.
In this way you won't silence spammers, you'll silence all the forum, in the end, the only one who would be able to post here would be theymos.
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.
Introduce the merit decay while having a signature at whatever rate and we will get rid of sig spam.
They can still post and if they want to shitpost for free that's it, hit the ignore button but I seriously doubt they will.