The best solution is a combination of reviewer reputation and compatibility matching. First, very few people are going to go to the trouble of creating a bunch of valuable reviews just to build up enough karma to be able to smear a particular business.
Then, even if someone were that motivated, if Yelp weighted the reviews I see based on how closely my reviews matched the reviewer's past reviews[], standalone smears would have very little impact.
[] Yes, I know that sentence should be taken out and shot.