I know I'm not in a position to give you any suggestions on this but how about punishing the offenders so they learn from their mistakes, there are far more individual willing to give in their best for the successful of the project. An extra amount of post isn't much of a deal irrespective of you not been paid for them.
PS: No personal attacks please lets keep the conversation professional.
There really aren't that many people willing to give their best though. Each campaign(the 1s that I manage) only has so many open spots per rank. No matter who is in the spot in most cases, users are only willing to post enough to get paid. They are not posting because they like a topic, they are not posting because they want to inform others about a subject in most cases, they are posting to be paid. If they're being paid for 20 posts, then they do 20 posts. Every now and then I get a guy who is paid for 20-25 posts per week and they do 40-50 posts a week.
It is easy to determine who is posting only to complete his paid post counts and who is actually involved in the discussion.
Those who actually involved in discussion are real posters, they will always review , what others have said about their point of view and care to reply them.
If you see a person posting 20 different posts each in different thread every week, you should believe he is only filling the shoes to get paid and do not care about the forum discussions.