The replies i have gotten so far have changed my initial thoughts. It is true that some people post to maximize their earnings. i was only expecting more from high-ranked members which shouldn't be so, everyone is good in different aspects of the forum and can contribute to the forum irrespective of rank or merits earned.
That's why sometimes a no-minimum posting requirement on a campaign can be a good thing to consider, but you can't stop someone from achieving a max post requirement. I don't think reaching the max post count is against forum rules, but when the quality is low or maybe very bad then it should definitely be reported.
But your discussion is now biased towards other things which may no longer be the main purpose for which you created this thread. You started talking about campaign participants who don't care about post quality but they rank high, so let's hear my suggestion: Report spam posts or low quality posts to the moderators, it can solve all your confusion.