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.
Maybe before, the signature campaign was really one of the biggest reason why most of the users are spamming. Yet look at how the signature campaigns right now. The current standards of most of the signature campaign that can be found on the services section(except for the ICO's or in the alt coins are still on this problem) is really high.
You are actually been seeing the rules quite some time for some of the known campaign managers in this forum. That issue should really be addressed into most of the signature campaign that can be found on the alt coin threads.
The other big issue for me right now is the bounty hunting aside from most of it are just scam it has also been dragging the forum standards down.