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After reading your post, I would like to say a few things...
The first thing is that I totally agree with you. We can't "complain" about payments when things like this happen. The second thing is that in this case the moderators cannot do anything, since although it is true that he has written 15 posts in a very short time, he has not broken any rules of the forum.
I think that in this case it is the manager who should take action, and possibly those measures could be a rule in which users must publish their weekly quota throughout the week, and for example expand the minimum of 100 characters per post. By expanding the number of characters in each post, managers will ensure that users develop and express their ideas and opinions better, or at least I think so.
About a week ago I saw this on Telegram:

That user only received payment for 9 posts, I seem to remember.
I think it is possibly the quickest and most effective solution for users who are trying to abuse to realize that they are doing things wrong.
I'll say something else, although some call me an iron fist again... When users do that and nothing happens, the overall campaign can degrade. Getting that user to change their behavior is one solution, and if that doesn't work, another solution would be to remove the user from the campaign so that someone else can do an acceptable and adequate job.
Edit: I will try to talk to the affected user.