You can already buy Twitter followers and Facebook likes, why not plugin reviews if it is possible? There should be a way to report suspicious entries.

If you see anything that deserves moderator attention then you can tag that topic with modlook. That includes reviews as well.

That said (and this is important!) there is no forum rule against anyone leaving a 5 star review for their own plugin or theme or their co-worker's plugin or theme. Don't go crazy with that tag, you'll get yourself into trouble that way. ;)

The review section is often looked at by the moderators like myself (oh boy, is that an understatement) and fraudulent reviews are caught and dealt with appropriately. If you see one that's abusive then report it. If you are not sure then please assume good intentions and consider leaving your own meaningful and informative review instead.