Yobit had a long run signature campaign earlier in 2016 to 2017 for more than a year (don't know if it really stopped as I frequently see some users wearing the signature) with the highest pay-rate (compared to actual running BTC campaigns considering also the BTC price at that time.
Yobit's old campaign was still up and running before this new one they're listed on
Mitchell's overview. Only few users continued wearing it because the campaign was having issues in paying out, I remember one of their participants saying they have to waited months before they were paid.
The only thing I can think of as a semi-solution is: forum members may not be paid for more than X posts per day - no matter the campaign. It would slow things down.
Another rule suggestion means going through another series of debates and another pile of work for the DTs. Don't get me wrong, I like your suggestion but the question is do they like it too?
If Yobit plans to add a rule on their campaign it should be the merit requirement.