So to my suggestion:
Amend the forum to hide signatures of any account that hasn't earned at least 1 Merit in the last 2 months.
This will bar them from getting paid to post and not have any negative impact on anyone making even the tiniest contribution to the forum. Nobody loses their rank and even if someone is away from the forum for a while they can quickly get their signature back.
I like your idea of hiding the signatures of the signature campaign participants. This will force them to make a high quality posts in order for the signatures to be seen. Although I see that mostly will abuse it by just giving their alt one merit and they will be paid but it can be solved because you can track the merits of a certain user.
There is currently no such thing as a "demerit". I'm hoping that the positive merits alone will be fine. I could add demerits pretty easily later on if necessary, though.
Maybe it's time for demerits to be implemented. I think that if a demerit subtracted merit from the sender too, or cost twice as much sMerit it would minimize abuse.
To be honest, I don't like the word demerit. Demerit if added in this forum will be abuse by many users. Lets say, I am angry with user X and lets say I will demerit him 5 times or 10 times not because he is a shitposter but because I am angry with him. This can happen with anybody that is why I don't like demerit in the forum. Just merit

In my opinion, maybe all of the bounty campaigns must have a minimum merit requirement and all of the campaign manager must put a minimum merit of maybe 3-5 to Jr member, 15-20 to Member, 105-110 to Full member and so on and so forth. In this way, it will be harder for shitposters to join in the signature campaign because they don't have enough merits to join.