I think a great fix/solution to spam posts are to do a manual review of the users' posts every time they request that their earnings in YoBit be transferred to their balance. Maybe set a threshold on the request amount so they have to hit a certain amount for a minimum so that members aren't requesting after every post. Members with spammy posts can get the rewards for those posts deducted from their transfer total, and repeat violators should be suspended for, I don't know, a 72-hour period or so. Repeat violators of the spam posts can be banned from the sig campaign.
I know that this introduces quite a few man-hours for a manual review process, but there should also be an (internal) option on YoBit that allows the reviewer to green-light an account of a member that posts constructively, thus allowing for automatic transfers to their balance upon requests.
Otherwise, I've reported spammy posts to moderators by clicking on the "Report to moderator" links at the bottom of their posts, and have used the reason "ref spam" or "sig spam" when submitting my report. I'm not sure if this affects your sig campaign after your posts have already been reported to YoBit, but I think it would be good audit sig campaigners' accounts to make sure that they are not posting irrelevant or spammy content throughout the forums.
Just my $0.02.
This is not a bad idea actually. If all what you said is done then this campaign will be spam free and there will be no spammers anymore, but it needs a lot of time and effort for doing so and i don't think the admin or the managers of the campaign are free enough to do so, maybe they should hire some other trusted and free users to do it for them.