If someone were to buy merit to participate in a signature campaign (or otherwise), and are a crappy poster, they will either get banned, will not be hired to participate in a signature campaign, or both. The buying of merit would serve as somewhat of a non-refundable deposit allowing them to participate in a signature campaign.
Practice has shown that your theoretical reasoning is wrong. The forum is flooded with shitposters who: a) Are not banned. b) Are hired by crappy or non existing signature campaign managers.