At Marketing Profs, CB Whittemore interviews Guy Winch, author of The Squeaky Wheel: Complaining the Right Way to Get Results, Improve Your Relationships, and Enhance Self-Esteem.  Winch, a psychologist,  offers up advice on how to deal with complaining customers.  The trick is to make the complainers contribute to the business, and not simply to ignore them.  In order to do that, Winch argues for making the complaint process more constructive, and reminding all employees of the value of criticism:

Businesses should educate employees down the ranks—especially frontline employees—about the value that customer complaints provide to companies.

First, complaints are a crucial source of information about potential problems with products, services, or procedures that might be causing customer attrition in addition to customer dissatisfaction.

Second, they provide companies an opportunity to perform service recoveries and engage customers in a dialogue while doing so. Companies that truly listen to their customers' complaints will gain valuable insights about customer needs and wishes. Companies can then apply those insights to improve the customer experience.

Read How an Unhappy Customer Can (Paradoxically) Help Your Business here.