As a frontline representative, what is the best way to deal with unhappy customers who have unsavoury remarks about the company you work for?

Don't take anything personally.

Dealing with difficult customers can be challenging. But if you handle the situation well, you may even be able to improve your relationship, and create further opportunities.

Make sure that you listen actively to his problems or complaints, and resist the urge to interrupt or solve the problem right away. Be empathic and understanding, and make sure that your body language communicates this.

Use feel/felt/found to empathise and get them on your side as a person. In this situation, if they see you as a person rather than as a company stooge, they will generally calm down and trust you more. 

http://changingminds.org/disciplines/sales/objection/feel_felt_found.htm