I'd like to see signature only shown if that user received X merit in last Y days. I don't know appropriate value for X and Y, but i expect it could somewhat reduce spam.
Let's make it 5 Merit in 120 days, which is what some campaign managers ask as a minimum to apply. That suddenly makes those 5 Merits worth 17 weeks of campaign payments. This will lead to Merit abuse.
I don't think you will be selected in any good campaign if you have earned only five merits in the last 120 days as the competition is quite tough out there. So practically having five merits doesn't let you in any campaign, although campaign managers do have a condition of a minimum of 5 (or 10 in some cases) merits to apply for a campaign.