Combating paid reviews

Anne R. Allen has a good post/collection of links about the paid-reviews scandal. She points out that certain types of gatekeeping (in her examples, the requirement that a book have a certain number of 4- or 5-star reviews) can encourage abuse, which I have noticed, too. That's a tough one, because people typically set these systems up with the hope of weeding out bad books, but then if the system is easy to game....