That's why Newbie and Junior signatures need to be removed completely. They then cannot get paid for this until they become a Member and earn ten merit. Ten merit is not a lot to ask, but it is if you're a bot or the worst of the worst of posters.
I think that would be the best option, and I dont think it would affect traffic so much. The forum would send the message that you can make money
if you make an effort to rank up and get merits. And ten merits is not that much, but a shitposter will never get them. There would be people trying to cheat the system, of course, but the situation would be much better.