Attaching a reputation to each critic and weighting their opinion on it could work.
Critics would be loathe to be discovered taking money if it dropped their ability to influence aggregate ranking (and those who send the money would be loathe to send it if sending it made it worthless).
It used to, the professional critics were mostly just those that were paid by newspapers or similar. Lately as in the last few years the gates were opened to basically anyone with a blog viewed by more than their parents. This let everything be manipulated much more easily.
Critics would be loathe to be discovered taking money if it dropped their ability to influence aggregate ranking (and those who send the money would be loathe to send it if sending it made it worthless).