Rewarding high-quality reviews is important. At the same time, we are trying to build a positive site where all contributions are welcome. So, rather than flagging bad reviews, we have two systems for rewarding high-quality reviews.
Reviews are attributed 'excellent' status via an editor rating system. You can read about excellent reviews here.
Any Publons user can endorse a review. An endorsement shows that the endorser considers the review to be worthy of their advocacy. You can read more about endorsements here.