The root of the problem doesn't lie with the signature campaigns themselves. Certain campaigns which I was in previously (and arguably now) promotes posts with good quality with sufficient moderation by the signature manager. They screen each individual participant before and during the campaign. Some campaigns just straight up don't care; the managers don't look at the posts before sending payments and accept every participant. If anything, signature campaigns should be restricted and I frankly don't mind that much if they are removed completely. Might sound hypocritical but I'm all for improving the forum even if it means that incentives to post will be removed.

Your solution won't work if the signature manager doesn't view each thread individually / see the flags on the thread. If you want any meaningful discussions, you should go to the serious discussions subsection. While its not as active as others, thats the only place where you won't see any sig spammers.