Disgruntled customers, whether having a justifiable reason for being so or not, have many ways to protest. Facebook, Twitter, feedback, forums and the likes of Trip Advisor all provide the opportunity for a customer to vent anger or dissatisfaction. Those on your email marketing list probably know their way around the internet.
Such comments are not something you can ignore whilst luxuriating in the five-star ratings of those happy with your performance. If someone comes up with a one star there should be some sort of plan for how to deal with it.
It might well be honest feedback so you should investigate the circumstances. At the very least, you have a dissatisfied customer. Whether the complaint is justified or not, it is still something you have to resolve.
The one essential is to wait for a while. Do not respond in temper, frustration or, probably the worst way, from moral superiority.
If one of your staff has made a mistake then it is normally best to admit it as soon as reasonable. Do not blame Dave in accounts. It is a corporate failure and you should accept the blame. Tell them, on the same medium, you are sorry that you have failed to live up to your normal high standards of service.
Explain that you have a complaints system that is easily found on your website.
If you are in the clear then resist the temptation of putting the person in their place. Apologise that your processes or procedures have not reached the level that the person anticipated, and then go on to explain, politely, why they are talking rubbish. Do not attack them personally or criticise them on public media as the tone of your response is what other potential customers will remember. They don’t really care about the details.
If you have done something badly then correct the error in a manner that is reasonable to both parties. It might be a time to be generous rather than precise.
Expect that you will occasionally receive poor reviews and criticisms on public media. Your response can be made into something positive and you can end up gaining sympathy and subscribers with sensible procedures.