In what I've begun to think of as the Sock-Puppet Saga, thoughtful people are presenting ideas to help authors overcome the reprehensible practice that has become known as sock puppetry—the posting of fraudulent, negative reviews on rivals' books on online marketing and sales sites through fake identities to destroy the prospects of the rival, while at the same time posting fraudulent positive reviews through fake identities of a chosen book.
Here's an article from Forbes Magazine in which the author has come forward with practical, useful ideas for fighting back effectively. There are common-sense suggestions, like not allowing anonymous reviews and requiring reviewers to show proof that they've purchased the book they are posting a review on. And there's an intriguing idea picked up from the way Craigslist is intervening to stop people from posting fraudulent information on that site. Finally, there's the bottom-line issue of everyone involved (including all authors) acting with integrity to stop the frenzy of scrambling for reviews in order to qualify for marketing opportunities on the sites.
Given the way the system's set up, that last one sounds a little like asking authors to take the hit for poor site set-up in some ways, but the point it important, nonetheless.