Fake reviews are just another form of marketing, like traditional junk mail. Making mail spammers pay postage doesn't seem to have changed the fact that most of the snail mail I get is junk, even in the digital age. Charge reviewers a quarter or a dollar to leave a review. If I was passionate enough -- one way or the other -- to leave a review, I'd pay it! I'll bet it would squelch the fake reviews overnight. Just tune the charge to find an acceptable signal-to-noise ratio.