The easiest solution seems to be stopping them from paying per post. Salty has the right idea.
This is not a good idea

1. Do you realize how many Bitcointalk users are earning money for their posts while they are unemployed? I know that many, many of them.
2. There always will be a spam somewhere - you cannot get rid of it just by disallow ads in signatures - sure, you will get out few spammers, but there will be more and more even with no adv in sigs.
3. If you will disallow ads in sigs users like me will be very sad and mad at you

because this is the way to spend very nice time @bitcointalk and earn money at a same time - I spend @bitcointalk about 3-5 hours daily - sometimes more.
And sorry, but if I will lost my job as a sig seller I will not be able to spend here so many hours ... because I will need to find a way to earn money
You might say to me "So you are posting just because of earnings?! Ban him!"
I will say to you "No.. I am posting so much because I love it - and I need money too so this is so great to earn money while posting!
I believe people like me can make @bitcointalk better - bigger forum ever!
So Bitcointalk is earning something from those Sig sellers too! Yes? Yes!
anyway... I hope @bitcointalk will never disallow ads in signatures ...
Well, if we cant find a suitable solution, signatures all together could be banned, or a slightly less radical approach would be to just ban paid sig advertising all together. If you are earning enough money by posting to support yourself financially, chances are you will be banned before your first payday. You are correct, there will always be spam, but paid advertising made the amount of spam go through the roof. Volume of posts does not equal quality of posts. If someone is posting a lot, but they are rubbish, they are making the forum worse, not better. A lot of vets who helped build the forum to what it is today are leaving and its just getting spammier. Our best course of action would be my original suggestion and have paid signature payout rates based on activity rather than post count. If you have 20x more posts than activity, your chances of getting banned are astronomical. We see those huge post counts and check through your last posts with a fine toothed comb. If there is a significant amount of junk, you are banned indefinately. Pre Signature Advertising, there were a few people banned per day, now they are a few peopled banned per 30 minutes. By doing it based on activity, paid signature advertising beneficiaries only need to post once per day, or 14 times per two week period, that means they can post 14 quality posts, rather than one word answers and faces.