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.
I do not agree to this idea because:
1) One of the reasons behind the popularity of this forum forum is that it provides an earning opportunity to new members without paying any money. There are various other websites that charge extra yearly fees for special ranks that are associated with privileges like signatures. Those forums will never be able to touch bitcointalk in terms of popularity.
2) Because of the above reason , a lot of new members keep joining the forum in the hope of getting accepted. They also make posts here and all this results in forum getting traffic. Indirectly, these members are also helping forum and are responsible for the advertisement slots being sold here.