When creating thread, another additional option should be available and that should be something like; No Signature
Why? Not every person wearing a paid signature is a spammer, and, people wearing paid signatures are technically paid for posting quality content, though I know bounty campaigns have changed the meaning of that, but technically the purpose of a signature campaign is what I said. And you cannot disable signatures in a discussion where quality content can be posted by users wearing paid signatures. And besides, the spammers are not always users with signatures these days, there are spam bots all around the forum who won't realize a thing no matter what you do.
So it is really not a solution to control spam at all.
signature spammers slowly realize that their effort to spam certain threads is not working, they will put their focus on other threads they can spam and this will create space for quality discussion at certain subs/threads.
So you basically mean that when the spammers realize their efforts are useless in signatureless threads they would find threads with signature showing features and spam them like hell and that would allow quality posts to be on top? Seriously? Don't you think it would encourage them to create many more threads of their own and spam every single section doing this only to fulfill their own needs?
This might be a good punishment mechanism to prevent unnecessary replies made by sig spammers.
This would only be the reason of a new wave of thread spamming all around the forum where the creators of those threads as well as the posters would be professional spammers of this forum.