Even if you detest review sites, it's still worth opening a business owner account on Yelp, Citysearch, and any other website on which your customers are talking about you. Registering generally allows you to correct inaccuracies, receive alerts when you are reviewed, and respond to your critics.
Just because you can respond doesn't mean you should. Anything you say -- in a private message, a personal e-mail, or even a voice mail -- could end up on the Web. If there's no way to respond to a review without being angry, profane, or aggressive, don't do it at all.
Apologize for what the customer didn't like, and offer to make it right. When Lauren Hart, owner of the Root hair salon in Phoenix, received a two-star review from a woman who didn't like her haircut, she wrote, "I am so sorry that you are unhappy with my work," and then offered to pay for a cut at a competing salon. The woman wrote an equally gracious response and upgraded the Root to four stars.
If you can't be nice to the reviewer, try complaining to Yelp directly. The site removes reviews in cases where there is a conflict of interest (for instance, if the review has been written by a competitor). Other grounds for removal include hearsay, hate speech, and attacks that are unrelated to the customer experience.
The Communications Decency Act protects websites from being held responsible for the actions of their users. And although defamation lawsuits against Yelpers have, in rare cases, succeeded in getting reviews taken down, suing tends to attract the ire of other Yelp users. If you decide to sue, be ready for more attacks.