A) Make It Easy For Customers / Clients To Give Feedback
If possible, put up a customer hotline for your business, and also allow the customers to give feedback anonymously (not everyone likes to disclose their personal information).
When they are able to remain anonymous, in leads to more honest feedback.
B) Thank You For Your Feedback
If a customer is giving feedback, even if it is in the form of a rude complaint, you must realize that you are benefiting from his/her frustrations. Do give some form of compensation as a nice thank you note no matter what the feedback may be so that your customers feel appreciated for their time taken.
In the end this will also show your customers that whatever their thoughts and comments mean something important to you.
C) Analyze Their Feedback
Is this feedback based on an isolated incident? Have you received similar feedback before from other customers? Is this customer being reasonable in their requests and/or suggestions? Will acting on this feedback benefit the organization as well as other customers, or just this one customer?
You certainly do not have to act on all suggestions, but you certainly should consider all of them with care.
Customer feedback should be used as just one of the sources of information in decision making, but never the primary source. There are times when you know your product better than anyone and your vision and determination will make it work despite an anonymous.