If the main goal is to stop signature campaign spammer, then i have another idea such as :
1. Signature limitation based on your merit amount, not your rank
2. Signature only enabled for those who earned x amount of merit within y last month
I doubt signature campaign spam is that big of a problem these days. Very less bitcoin signature campaigns and strict moderation including merits have cut it down. Extreme amounts of spam come from the altcoin bounty campaigns including massive account farming. Some merit law should definitely be implemented in those sections.
When i'm talking about "signature campaign spammer", it included altcoin/token/bounty signature campaign.
If it combined with more strict rule (e.g. only member who've earned 100 merit or more who have signature privilege), IMO it'd be very effective to stop spam.