The reality of this forum
The obvious solution is to remove signature campaigns all together. This will undoubtedly get rid of the people who are here for the wrong reasons. But Theymos and mods know this will absolutely kill the traffic to this website and ad revenue would absolutely plummet.
A lot of the boards don't even have real discussion. I guarantee a lot of these guys just skip to the last page, post something that is somewhat relevant to the topic and dip to the next thread to fulfill there bounty quotas.
I agree with you that very often signature campaigns are the reason for shitposting, because people just want to fulfill bounty quotas. But at the same time, signature campaigns can be a motivation to write better and more qualified posts. The problem is not in the existence of signature campaigns, as such, but in the fact that they put forward different demands on their participants.
There are campaigns that take everyone in a row, but there are those which set a high bar. And if the participant doesn`t reach it (not enough merits, bad history of posts, etc.), then he doesn`t get into the spreadsheet. Perhaps, instead of eliminating the very phenomenon of signature campaigns (which will have a bad effect on the website's traffic), it is necessary to establish certain rules for recruiting participants. If the bounty breaks them and reduces the set bar, then the admins will close it.