Removing signatures for Jr. members can do the work because they cannot reach Member rank without earning Merits, and if someone can manage to earn at least 10 Merits, I believe they deserve to have the right to wear a signature.
It doesn't even need to be 10. If you set a 1 merit requirement to reach Junior Member, that would probably stop >95% of the spammers. I don't think that's an unreasonable requirement.
Or, how about this: You gain 5 characters of signature space for every earned merit.
