Signature advertisers just need to be restricted on the terms they provide, the reason for all this spam posting is purely to boost their post counts because of the minimum requirements these people have, if you forced them to pay on a time basis rather than post count it would force them to either leave or adapt.
I don't really see a way to do this effectively though, not really any way to force them to do anything. We can ban them and they'll just add a link to the sigs they're already buying to an offsite offer.
I'm also pretty sure Ritz, Stunna, and the rest don't give one hoot about people who get banned over this, because banned users still have the signature, and it's also retroactive, so they are getting exposure they don't even have to pay for. Really it's a bonus for them.
One solution would be to remove post count all together. Then it's up to Stunna and others if they want to pay per activity. It would be the best option and best decision we can make. It won't reduce all spam. However, it will reduce a large amount of it and bring the noise to signal ratio back down.
Something needs to change this is becoming unbearable and I know the staff are doing what they can, but it's out of their control. As Bitcoin becomes even more popular there is going to be a lot of new users who are looking to make small amounts of money and the first place they will go is to the signature program.
Or just ban it all together. The individual spamming would get banned, so in theory the person who is paying and encouraging it should have their 'advertisements' banned.
(I believe Badbear has already suggested this solution)