I mean it's no secret that people shitpost for sig campaigns. I think if we put a cap on their earnings, for example, once you've earned $5, you need to stop posting.
This is a forum where people are free to share their opinion whether it is consider a spam or shitpost or not and who the hell wants to stop posting if they know that they are earning thru it?? Its like "You don't like to have Gold". Who doesn't want money 
. They will do anything just for money 
Also, impose bans on account sales on Bitcointalk, so people don't buy accounts to bypass restrictions on Newbie users that are there for a freaking reason
I think the negative trust of the sellers is good enough to say that these persons are not trustworthy therefore, they must avoid them at all costs. At this moment I don't see any solution to this spam problem rather than removing the signature campaign itself but for sure, the number of posts will decrease if that happen.