Since you can’t please all the people all the time, it is almost inevitable that this will happen to your business at some point. In many cases, a good faith attempt to resolve the issue will work; in the remaining cases, you may wish to pursue your legal options. Unfortunately, they aren’t very viable.
North Carolina defamation law allows you to recover damages for false statements when the person making the statement either knew the statement was false, or did not take reasonable care to determine the falsity of the statement. North Carolina recognizes three types of defamation: defamation per se, where the statement is obviously defamatory; statements that are open to two interpretations, one of which is defamatory, and the other is not; and defamation per quod, where the statement is not defamatory on its face, but may become so when combined with outside innuendo or explanation. However, all three types of defamation are subject to an exception created by the First Amendment to the Constitution: opinions are not actionable, only factual statements.
In practice, this means most online comments are not defamatory. In fact, the more incendiary a statement is, the less likely it is to be actionable, as courts routinely find that statements full of exaggeration and hyperbole are opinions, not facts, and the typical online complaint is not noted for its reasoned, factual basis. A comment like “This is the worst service I have ever received,” is clearly an opinion could not be the basis of a defamation suit. Even the statement “they’re a bunch of criminals at that place,” might not be actionable, especially if it is surrounded by other statements which may be framed as exaggeration or opinion—context is always important in a defamation claim.
Given the First Amendment exception and the nature of most online complaints, you are almost always better off trying to resolve the complaint amicably, rather than bringing the courts into the dispute.