Glad you would ask. Here are a few things you can do.
1) If you know the individuals who left the reviews on Yelp, you might contact them and see if you can resolve the problem and get them to remove the review.
2) Respond in a professional way to the reviews that are there. Don't be defensive. Look for ways to reassure - like a great service guarantee that isn't advertised or a quick mention of your patient surveys that say 98% (or whatever the number is) say they are satisfied. Be humble, not defensive.
3) Focus on Google reviews. They are the ones that matter most. Yelp is becoming more important, but it is still not anywhere near as important as what is in Google and Yahoo.