How about approaching the solution based on principles applied in real world governance (with the exception that it's codified and decentralized)?

For example, if the "jury of peers" system were applied where "trials" would be the random spot checking of posts, given to random members in real time, to be completed within a specific time limit, with some amount of discussion among a small group that must come to a consensus on the approximate level of quality of a post. The reward for "jury duty" would be based on the how close the group's consensus was to that of other juries.

Abuse of the system is prevented since premeditated collusion isn't possible within a group, let alone between groups.