I can't say I'm that sure about the effectiveness of this. Let's take one of the most notorious campaigns for spam as an example, YoBit. A large amount of members removed from that campaign are below 'Member' rank (AFAIK). It is not difficult to get the potential activity for an account to that level, especially if said spammer is managing several accounts at the same time.
Due to this, I don't think that this would be too much of a problem for people dedicated to spamming due to their ability to switch accounts should a negative repercussion happen to one. This would be even less of a problem if the implementation included staged punishments (3 days, 7 days etc) as if one account had their signature removed they could move to another account, then move back once said other account account had their signature removed.
I think you haven't really thought about the bigger picture here. You're working under the assumptions that everyone has multiple accounts to spam from (these are usually identified by staff members though). Let's assume that a person has two accounts of which one is a rank that is not sufficient for the sig. campaign. His main account gets banned for spamming; there will be a time period where they either wait out their ban or wait for their other account to rank up so that it can join a campaign. Additionally what should be implemented is that at the second warning (i.e. ban; after this one you often permanently banned) the signature would get removed permanently from the account.
This would be more effective on members of higher ranks though. It is still better than doing nothing as there will never be a perfect solution to this problem aside from banning signature campaigns.