It's an interesting idea. The question is, how do we determine how many merits someone needs to get before a signature appears?
Based on spam rate, Merit rate, Merit-to-post rate, posts to merit posts rate...etc
Sounds nice but I'm pretty sure in reality a lot of them will manage to get enough either directly or by trading it.
Even the red-tagged 1xbit spammers are getting 1 or 2 merits, so for the more advanced spammers capable of writing 5 not 2 lines of text 1 merit will be reachable. Also, I don't think the WO members will be happy with that, that topic will be flooded with merit begging.
Most of us will fail in this task. Suppose, for example, that the members who participate in the signature campaigns are 500 members, and with restrictions of about 20 Merits per month, this means 10K merits per month.
Merit sources generate 33989 and therefore it will not be a difficult task, especially since what you will lose is the signature (for a week/month) and not your account.
This will also move the forum as a whole as it is rare for members to be removed after they join signature campaigns, such restrictions will make it easier to remove a member and thus add more members.