The problem with this is if we open publicly and have such instant payment, you know some people would make accounts up to minimum requirements, posting bad quality posts, then bot spamming hundreds of posts with randomly generated after joining campaign, right before a CRON, to then immediately withdraw.
Your payment is set to weekly, right? So that should not be a problem. If you want to control the spam then check the post manually a day before the payment or better yet daily before the creditting schedule. Since yahoo is already the manager, he should be able to monitor everyones post quality thoroughly.
Another option is to set a minimum character requirements gor post to be counted and limit the boards your user can post becase there are several boards here which receives a lot of spams from coinroll members for the past few months now.