Maybe the focus should then be on reworking who's allowed to post and how many times you're allowed to post in the different forum account levels, for example a set amount of posts per day as a maximum. The signatures should remain unchanged, for those advertising and for those who don't.
I don't believe spam stops because people have lesser chance of earning money from it, but if the amount of posts per day is regulated and spam posts reported, people won't waste their daily posts on spam. Of course the biggest restrictions would be against newbie accounts, many people would already not risk losing their full/sr/hero member accounts, as theymos himself said.
We have witnessed spam start because of the chance of earning money from it, so we are trying to undo that. Restricting how many times per day you can post is more restrictive to everyone, than changing signature styling which is just restrictive to paid advertising.