I have had this happen to me a few times. My usual response is to send a reply asking for evidence that the negative review is fraudulent or intended to harm business rather than being an honest review.
90% of the time I will hear nothing back, when I do hear something back it is often blather that I can respond to with a few sentences. Only once have I taken a review down when I received a complaint that the review was posted by a competing business. I dug a little deeper and emailed the original poster, who was very petulant about the insinuation, until I looked deeper and found that particular guy had posted all negative reviews about that particular business, but was commenting/reviewing another business none-stop.