You could make a formal request to all of your customers or clients to not leave reviews of you or your offers, but first amendment rights being what they are in America this would probably guarantee that you would get reviewed only by your worst customers. 

If your concern is getting bad reviews, don't worry about it and embrace this site as a way to more easily participate and even manage the conversations about your brand, than you could on Facebook or Twitter.

More importantly this site has the potential to get your more customers by leveraging the authentic recommendations of your existing ones.

The best way to protect yourself against bad reviews is to keep improving your offers and to ask your best customers to leave reviews.  They'll be happy to do it.