I think if we put a cap on their earnings, for example, once you've earned $5, you need to stop posting.
I think the forum admin and moderators don't have time to monitor all signature participants and implement that rule.
I'm not sure if you are targeting BTC-paying campaigns in your post. Altcoin bounties are paid at the end of the token sale and there's no way to determine the real $ value they earned during a week or a month.
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This isn't a freelancing place where people will stop doing what they do once they get paid. Limiting someone to post eliminates the whole purpose of a forum intended for discussion, regardless of how gruesome and clusterfuck of a spamhole the forum is.
I concur.