mrsjohnson, because you inquired in another thread about "no negative review" (non-disparagement) clauses, I thought I would provide information on them here for you (I provided much of the same information in the other thread).
Forum member LdiJ mentioned in the other thread that she would be likely to avoid a business that had a non-disparagement clause, saying she would question the reason behind the business needing one. Her reaction is shared by many. These clauses are not liked by consumers - so much so that California enacted a law that makes clauses that restrict the ability of consumers to post a review illegal. There are civil penalties for companies who try to enforce the clauses. And there is a bill in the US Senate, "The Consumer Review Freedom Act," that would also prohibit the use of clauses that prevent consumers from communicating to others about the goods or services of a business.
Yes, negative reviews negatively impact a business. That is the reality. A paper written by Michael Luca for the Harvard Business School, "Reviews, Reputation, and Revenue: The Case of Yelp.com," was based on his research into the impact of reviews on businesses (restaurants), and the research showed that each rating star on Yelp equated to a 5% to 9% effect on a restaurant's revenues.
When a review is not only negative but also false and defamatory, the business (or individual) negatively reviewed has the legal option of filing a defamation complaint against the consumer who has written the review. But preventing a consumer from writing the review is a restraint on speech and this country does not like prior restraints too much.
Here are links to access the Yelp paper by Luca, and to the text of The Consumer Review Freedom Act:
"Reviews, Reputation, and Revenue: The Case of Yelp.com:"
http://hbswk.hbs.edu/item/the-yelp-factor-are-consumer-reviews-good-for-business
The Consumer Review Freedom Act:
http://www.commerce.senate.gov/public/_cache/files/182837c8-cdc6-4eba-8c1f-fce3df22a26f/863E0F2A79287FB92CDF9975761E6C64.consumer-review-freedom-act.pdf
I know you have been mulling over whether to file a defamation suit against your negative reviewer. The customer seems to have written his review with the sole purpose of harming your business. I will post back later (probably not until Monday) with what you will need to support a claim in South Carolina - which is the state where I believe you are more likely to have success with a defamation suit over a negative review. But you will want to review all of the facts with a defamation attorney in your area (or in South Carolina) and then weigh carefully the pros and cons of a suit.
Your goal should be to preserve your business and its customer base, and recover from any harm the one negative review might have caused. I am not convinced that a defamation suit against a customer is the best course of action for you. Like the non-disparagement clauses that customers appear to hate, they would probably not look favorably on a business that sues a customer over a negative review, even when the review is false and defamatory. I think a lawsuit should be a last resort, at any rate, and only if you and your business continue to experience a negative fallout from the customer and his review.