There isn't anything you can do really, it's all part of business. You can't make everyone happy... but you can ask them to take it down - but they don't have too - it's all part of freedom of speech (at least here in the states) - now if it's nothing but lies it could be "liable" but that has to be proved.
try to calmly explain the situation or reason instead of raging, i think this is best policy beside this there is nothing you can do. there is both +ve and -ve comments , one cannot make everyone happy.
you can not make every one happy but you can try to make him/her happy . you can ask his reason why he gave you a bad response and you can resolve it or simply remove that comment
That is true, there is nothing you can do. That would be like asking Yelp! to remove bad reviews. The best way to stop this is to keep all of your customers happy!