Fair rebuttal only makes sense. People understand that some customers are unfair and angry. Everyone, in almost every possible job scenario has had to deal with them.
At the same time, signing up for quality profile websites as another poster mentioned, and ensuring that you register and complete local listing websites can really help to reduce the exposure of the negative rating.
Another good tactic is to simply ask other satisfied customers to post good reviews where the negative review is posted. Sometimes having a negative review, in a sea of great reviews, is a good thing.