You're working under the assumptions that everyone has multiple accounts to spam from (these are usually identified by staff members though).
If someone were dedicated enough this could happen easily, I may just be overthinking it however. I realize these accounts are usually found by Staff rather quickly, though there are always exceptions.
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.
I was more assuming along the lines of said user having a backlog of accounts with potential activity, therefore ranking up will not be a problem. Posting once or twice per two week interval would not be difficult in order to build such a backlog of potential activity for the future. Posting a larger amount in the future to upgrade the account's rank to 'Jr. Member' in order to join YoBit, for example, would not require too much effort on the user's part either.
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.
Absolutely, that would solve a lot of the problems I was trying to outline. If this were to work it would have to be nothing more than a warning, and it would have to be strict.
This would be more effective on members of higher ranks though.
Yes, however it seems to be fairly rare that a higher ranked account is banned due to signature spam (unless it was sold recently). If this were the case and it was used to mainly pinpoint higher ranked accounts, I think it would be a waste of time implementing this for perhaps the same outcome as a simple PM.
as there will never be a perfect solution to this problem aside from banning signature campaigns.
At least not one which wouldn't require all staff to be extremely active/more staff to be taken on.