This is an increasingly common activity among well-funded businesses who don't like reviews that consumers leave for them. Here is an example: http://www.citmedialaw.org/threats/route-60-hyundai-v-alascio
One of the ways you can protect yourself is to let the Citizen Media Law Project know about the threat. They likely have attorneys in your state who can help you.
As a general legal principle, truth is the ultimate defense to a defamation claim. However, in the consumer review context, opinion matters as well. "Under the First Amendment there is no such thing as a false idea. However pernicious an opinion may seem, we depend for its correction not on the conscience of judges and juries, but on the competition of other ideas. But there is no constitutional value in false statements of fact.” Gertz v. Robert Welch, Inc., 418 U.S. 323, 339-40 (1974).. An alleged defamatory statement “must be provable as false before there can be liability under state defamation law.” Milkovich v. Lorain Journal Co., 497 U.S. 1, 19 (1990).
Therefore, unless you made a false statement of fact, a defamation claim against you will be exceedingly weak. However, you need to consult with a Georgia attorney, as defamation is a mixed creature of state and federal law. The claim is defined by state law, but federal constitutional principles will come into play in your defense.
Truth is an absolute defense to a defamation action, such as libel and slander.
This might be a tactic this particular business uses in an attempt to intimidate individuals who have posted honest, truthful, yet negative reviews. If true, this is a shameful business practice. Consider reporting the business' actions to your local Better Business Bureau office or even your local newspaper or television station.
Disclaimer: This does not constitute legal advice. No attorney-client relationship has been formed through this answer.