A few ideas cropped up in my mind.

1. A rule like - those who dont receive at least 5 merits during their posting period of the entire campaign will not eligible for payment - for those which are paying in bulk once campaign end.

2. Some campaign are having a minimum number of merits in order to enter them. It would be great if these people are also evaluated in future about their post quality by looking at the number of merits gained while participating in that campaign.