I've been reading meta for some time and I see lots suggestion about signatures and how to reduce spam and so on.
I couldn't find data on how many users are participating in various campaigns, I think that number might be 100K, maybe higher.
What do you think about
forcing users to pay for wearing payed signatures?
Why don't you charge
few bucks for it? It doesn't need to be big amount of money, it can be around 10-20$ monthly. Discount for larger time period could be included.
Imagine it, 100,000 users will pay
10 - 20 million dollars every year just to participate in campaigns!
You can even donate some money to charity

IMO that would be great way to monetize forum and reduce spam.