Many websites, from user forums to commercial sites with ratings systems, have this problem: how to vet the content users are putting on their sites to avoid being involved in libel suits. However, my experience of several forums would suggest that policing structures quickly evolve, whose main means of punishment is intellectual disgracing (often expressed as "RTFM" or "Google is your friend").
I have also wondered whether online shops such as Amazon manipulate their reviews. Again, it seems they don't, in order not to cause such rumours among their customers, although IP-range and cookie based action could circumvent the problem of people checking their own reviews still exist. Note that I am merely pointing out the technical possibility of this - Amazon has enough negative reviews on some items to suggest it's not greasing its wheels.
It does strike me that in some sinister "brave new world" (just read William Gibson), content transmission will be policed by the control freaks hired by forums and other websites to suppress the undesirable. To the best of my layperson knowledge, there is no legal requirement for private companies to give accounts of such content censorship. I do wonder, though, whether Amazon's terms and conditions have anything to say on the matter. Perhaps we should request this.