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.
That is shifting the spotlight onto the wrong spot. Aside from it not even been feasible to control (income external to the forum, very often in tokens, with no real market value during the campaign, and relayed months after the campaign ends), the problem is not on the amount of posting, but on the nature of the content.
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
The issue is that they are difficult to prove. People openly transact accounts to some degree here, simply because it is not explicitaly prohibited (although they may likely get trust tagged for it). If it were to be prohibited, people would hide their activity more here, but it would still go on elsewhere.
Proving that an account has changed hands is pretty hard, and even though I personally would prefer an explicit prohibition (simply to discourage in a more adamant manner this sort of activity, and to have a rule to be based to report them), It does seem that, more than proven cases, we would have to face likely cases, and banning on likelihood rather than on proof is probably what deters prohibition.