It's not really a question but doesn't it kind of seem like a problem that users are basically rewarded with higher ranks for post count even when they don't actually contribute anything? I'm probably biased because I'm basically a lurker on here but it seems like until you hit that merit requirement there's a real incentive to make as many low-quality posts as they can get away with. It's one of the things I don't really get about the forum.
I'm not quite sure of what are you asking, but maybe you are mistaken in one thing: when the merit system was implemented, the people receive a determined number of merits according to the status they already had. For instance, If someone was already a Full Member, he will receive 100 merits when the system comes, that's maybe the reason why you can see many users with 100 merits and tons of posts in their count: if they are shitposters they will remain with their 100 merits forever, for, as long as they don't contribute to the forum it is fairly improbable for them to gain new merits.
Maybe you can just check the date of their accounts creation: if they are from 2017 or before probably they got automatic merits when the system was implemented; if they are pretty new and have had tons of undeserving merits, probably it is a case of merit-farming (against the rules) and you can report them.