But we can play our part by reporting the posts made by the spammers in that campaign. Since most of the posters in that campaign are not well known for their writing skills, their posts most likely will get deleted if reported by one or more members of the forum. Once the posts the deleted, the participants won't be paid and hence, people will be discouraged to join this campaign.
I believe it's a classic reasons when forum/ admin really don't moderate scams including participants promoting scams like the campaign the OP mentioned. Not many people care about the report to moderator button when they find that campaign participant posting spam on this forum, but there are only a few people like suchmoon who actually do it by the thousands.
If negative tags don't solve the problem of people continuing to promote this campaign, is it possible to expect a signature ban for each participant? Is this signature ban also a moderation of scam?
Temporary ban, signature ban, and permanent ban have different cases. The scam is not moderated, and that's why I didn't choose "YES". IMO, most of the ban are made because the user violates the forum rules due to the continuous spam that gets his posts deleted too much and and most of the others due to plagiarism and evasion ban. Since you have declared yourself a spam solver on several occasions, then continue your good work of warning those users and maybe some of them will be banned for spam as well.