Bad Apple in your staff

I cancelled my contract last week on the 11th and paid the same day the last amount due. The agent who handled my call said I must phone next week to get my cancellation letter. Today I phoned, it being next week, to get a cancellation letter and the agent I spoke to was extremely arrogant, rude and said she will put the phone down in my ear, she did not want to listen to any reason off me explaining that I have already cancelled the contract. This agent wanted me to pay another monthly fee of almost R500 because I phoned this week and not the day after my cancellation date to get a cancellation letter. Common sense should be required for all your staff, why on earth would I cancel a contract pay the fee and then just go on paying monthly. I asked to speak to a manager and she refused to transfer me. I will not use you again, this is how you loose clients.