I don't think acting on campaign managers would work, they'll just stop posting here, or use anon accounts. While using anon accounts now would be a red flag for a sig campaign, it would eventually become normal and accepted, similar to how it's the norm to see a one post newbie selling a hero account.
I think it could work or would actually be quite effective. Sure they can still operate from outside the forum but losing access to their account is very bad news for them and their business. I would like to think that Campaign Managers will just take the hint and start actually doing their job and get on top of their campaigns or find someone else to do so rather than risk losing access to their account for x amount of time. If people start evading bans then they should just be treated the same as any other ban evaders and it shouldn't be hard to spot.
Something definitely needs to change though, this isn't a problem that can be solved with more moderation. For bans to be effective in countering sig spam, the number of bans would have be much greater than it's ever been. Even during the last "ban wave", some of the borderline users I was watching were saying "Well I might get banned but oh well, gotta get paid". The campaigns may not care that users that have been here for years are being banned, some permanently, but I do. I don't want to see that many people banned, ultimately it's not good for anyone. A sig spam ban that removes their ability to use a signature (and avatar, and personal message) would solve the problem of needing to ban users, but it wouldn't solve the sig spam problem.
I don't think we need more moderation, but the campaign managers can do that and that's what they're getting paid to do. I think we certainly need to put some sort of pressure or encouragement on campaign managers to actually do their job and in doing so it cleans up the forum and takes away stress from the staff. Imagine if every campaign manager actually bothered to check the users posts before they allowed them to sign up? The poor-quality poster would have no where to go and be forced to improve their posts and if it becomes common knowledge that people aren't going to get paid for poor contributions then that should have a big impact on the quality of posts, but when users realise they can post whatever they want and get paid automatically for it there's no reason why they would improve.
I still think an ignore signature button (with filtering to filter all signatures of that type) would be the best option, with an autoignore at a certain threshold. See a lot of spammers with a certain signature? Ignore it, and if enough do then that company has essentially had their signatures removed from the forum.
I think there should also be other measures put in place too and the hide signature button would be good but maybe giving mods or certain staff (Globals or Admins or even dedicated signature mods (which I would happily volunteer for)) the ability to ban/remove a persons signature for x amount of time would be another option. I think that would be better and less harsh than a ban whilst sending them a message that they certainly need to improve their posts which they could do in the meantime. Maybe it might even be a good idea to remove signature and Personal Texts altogether for certain ranks like we do with avatars and any member under Full can't have one because most of the poor quality posts seem to come from newbs/juniors/members who are too young or naive to know what's acceptable here.
I think an ignore signature feature would be an alright idea, but that brings up a point from me. So even if we have the ability to block signatures with certain words or tags or whatever, that would be great to hide annoying sig ads, but I'm not sure it would cut down on spamming. I'm sure there would still be users spamming the forums to get their signature campaign payouts, and the signature hiding feature wouldn't stop that. You'd still see the user's spam posts, but not their signature.
If every campaign manager checks before they're accepted, occasionally during the campaign, and, vitally, before they get paid and not pay those who are making poor posts then in theory the problem should solve itself. If people quickly realise they're not going to get paid then they lose the incentive to spam and drastically up their quality or they're just wasting their time otherwise.
This seems like a really tough issue, but I'm sort of leaning towards what hilariousandco suggested - banning users and campaign managers who can't keep their campaign spam under control. A full post and PM ban on a campaign manager probably wouldn't be a great idea, but if they were locked to only being able to post in their signature campaign thread for a week or two, I think that would be a fair punishment. That way they could still manage their campaign, but wouldn't be able to participate in any other area of the website.
This kind of defeats the purpose of the ban to me. It's like somehow 'banning' a spammer but letting him still post in certain sections. We could start leaving negative for campaign managers who don't do their job but I can't really do that now as it would almost certainly be a conflict of interest as I'm offering to run them (and I don't really want to leave negative for such a thing) but hopefully we can get the message across in other ways.