Even the best companies sometimes have negative comments posted about them. Dealing with these negative comments can be stressful but ignoring them can be much worse.
Your first step is to acknowledge the persons right to complain. If a customer has an experience that is not what he or she wanted or expected, the company should be notified. Most people when disappointed, just want to complain to someone and they want feel as if they are being heard.
A sincere apology goes a long way in resolving a conflict. Saying "I'm sorry" for the situation, the problem, the experience,etc while asking for more information can make the situation better immediately.
Many complaints revolve around company policy such as returns. Every company has a right to define their policies and you may need to clarify the policy that is relevant to their complaint. You should do this clearly with the reasons for the policy and how it benefits customers.
Asking "How can I help?" is probably one of the best responses. Don't try to second guess what they want as a resolution. Listen. Letting the user tell you their expectation lets the customer feel as if someone is listening and helps establish credibility for you and your company.
If their solution is reasonable and within company policy, make the complainer happy. Just do it! A reasonable solution goes a long way to turning a detractor into a fan.
If the customer's expectations are unreasonable, its appropriate to step back. Offer a solution that is fair and reasonable. Let the customer know this is all you can do. If he is still unhappy sometimes you have to walk away. If you have tried to appease the customer in an honest and fair way, others will see that the complainer is being unreasonable.
Some policies may need to be changed or have out lasted their usefulness. Negative feedback can help you evaluate how your policies stack up against your competition and determine if any new ones need to be added.
Monitoring your online reputation is an important facet of your digital strategy. Listen and respond. Ignoring problems doesn't make them go away.