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.

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 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.