What would you do to handle negative comments from clients, most especially when they express anger, frustration or hostility?

by showing my warm, friendly and professional character i can control the conversation , in same time listening skills is mandatory 

FIrst try to find out the reason of their frustration and if possible ask customer how can you help them in order to solve his issue

You need to listen to them with empathy- i.e. you have to place yourself in their shoes. You have not just  to listen to them but understand and comprehend why they are angry. That is, you have to understand the underlying causes behind their anger and aggressive reaction. Let them blow their steam first, so that they are a bit calmer when you start talking. You have to pacify them. Do not reciprocate anger with anger. Be cool and calm yourself. After that, you have to come up with a solution to the problem. The solution, of course, depends on the problem. Make sure the client leaves satisfied. Never let him go with his problem unresolved or boiling with anger. Without your client, your business can never survive.

Listen them carefully, show sympathy make them feel that they are right this will calm them down & in favour help them what could possibly best solution you can offer with current circumtances.

for example:

Special discount on product, free service for  maintance etc

The secret of success is to remain calm. Show respect and empathy to the customer by listening patiently and then put forward your point of view. Always let the customer know what you can do for them rather than what you can’t do. Offer a possible solution and ask if the customer is happy with that solution.