How will you deal with a narrow minded person?

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Ok I'm so pissed of with this kind of person, I'm a customer service associates and I am well trained to handle such any sarcastic client. Today I have encountered the most arrogant and narrow minded person in my life I almost loose my patient to him but still I remain professional and stick with my code but this person is not just one there are four of them and they are mocking on me they are not our client they were just representative of our client. They are relatives and they want to refund the money that our client pay they are keep saying that money is intended to be used for buying a new motorcycle of their brother but our client invested it in our company because they say that we hypnotized him and then forced him to sign the contract. Wow that ridiculous this is not just the first time we have encountered this kind of allegations it was started on a blog of someone who had been an ex-employee of our company and try too create a blog to ruin our companies credibility and he succeeded in doing it so. But this person doesn't have the right to cancel a plan without any authorization letter from our clients, our clients are already out of the country and doesn't have time to discuss this matter so he send a representative which is his brother in law. But as you may all know no one could cancel anyone's subscription with out the any valid proof from the clients only a written document with signature can do this. But this person are very narrow minded they can't understand a simple law. So, how would I supposed to deal with this kind of people?