Easiest way is to disable all signatures for all users and that will repel 99% of the signature spammers. No incentive = no hollow forced posts containing 100 of characters saying basically nothing.
Well, that's the only way to solve the problem for sure and I'm not against it. If these new guidelines don't work then I can't see any other option really. Campaigns that do little to nothing to monitor/curb spam after a warning will be having their signatures barred from the forum by an admin so we're halfway there to that. Hopefully campaigns will just start only accepting quality posters but for those that don't they wont be allowed to advertise here in such a way any longer.
How much traffic would the forum stand to lose if there was no more incentive for posting? 50% or more?
And that's the dilemma here. I'm sure it would be much more than 50% but 50% of people who are only here because of campaigns wont be missed, but even 'great' posters may eventually dwindle and leave as getting some money for being here is still a massive plus and will inevitably be what keeps some of them as active as they are.