Self-policing can work but I think the community is lacking some tools to do it properly. Merit seems to be a step in the right direction. I would like to have some visibility of how "report to moderator" works in the backend, e.g. which reported posts get deleted, which don't and why, etc. That would help to make post reporting more efficient. Even just seeing which posts have already been reported would save a lot of time.
Beyond that, asserting control over sig campaigns would require some sort of trust-like or merit-like structure where only approved campaign managers can do it and then can be held accountable by some metric. Otherwise they would just manage the campaigns outside of the forum. If theymos was willing to establish such structure the rest could probably be self-policed. Unapproved signatures could be reported. Shitposters carrying approved signatures could be easily reported to campaign managers and said managers could be penalized/removed if they fail to take action.
Merit works in some aspects but it only works so far. People with a lot of multiple accounts or "friends/classmates/cousins" still easily abuse it as do those that just trade merit between themselves. Also, a lot of these shit campaigns will gladly pay Juniors and even Newbie members for whatever crap they manage to squeeze out so merit/rank is irrelevant to those that do. This shouldn't be acceptable but if there's no rules or repercussions against it people will continue to do it.
Running signature campaigns off site would be an inevitability, but we had already thought of that and that's why we were going to blacklist their signatures as well if a problem campaign had been warned and then subsequently banned. We even got theymos to agree to it but ultimately it just never happened. It would require direct input and action from theymos every time but for whatever reason he didn't respond to requests for certain campaigns to be blacklisted. There's only so much we can do without his blessing or ultimate action and things like the signature blacklist are useless if he isn't going to enforce it.