The only other suggestion I can think of, is something I've suggested before: only allow signature campaigns that pay in Bitcoin. The ones paying in made-up tokens have no real cost for the ICO, and thus don't mind "paying" for spam. If the campaign pays in Bitcoin, at least they have something to lose.
I understand this point of view, as those token campaigns are really the main problem regarding spam.
However I think banning this payment method would be against the forum philosophy (this was the best word I could think of), almost a censorship and a way to control people' activities here. Even scammers are not deleted, but just tagged.
So if you use signatures for advertising, what are your suggestions for forum improvements in that area?
What I think would be a great addition is to add a high merit requirement to wear a signature. Something like 20 or 30 merits at least.
There are lots of users here with legendaries/hero accounts (probably hacked/bought) with less than 5 earned merits and are spamming the forum wearing signatures. I don´t think they should be allowed to do so. Maybe they could buy a Signature Membership like Lafu suggested...