How do I combat an inappropriate online smear campaign of a small business?

I am the owner of a small pet care business. We recently held a cute pet photo contest on our FB page in which an individual in another state (not a client of ours), potentially attempted to win a significant prize by using spambots and paid accounts to gain votes. We discovered through online research that this person may have done this many times in a variety of contests, and that he may be getting away with winning a lot of stuff this way. When confronted with this, the individual began a smear campaign of our business online, including posting negative reviews of our business on Yelp and Facebook. We are an honest, hardworking, and quality service. These reviews make us appear quite negatively to our clientele. This one individual threatens to take us down with a smear campaign from another state, having never even used our services. Admittedly, there was no verbiage that said spambots were not allowed, but we feel this goes without saying? What can be done?