Debited my account without permission

I had cancelled my contract with MTN, and was promised it was cancelled. On the 20th of October I then receive airtime and bundles, I call MTN again and spoke to a guy called Unathi, asking him about this airtime and he assured me to use the airtime and that it will not be debited on my bank account. Now end of the month a debit order goes through my account. I called MTN to inquire about this and they tell me I have to pay for what I used. I told them of my previous conversation with this Unathi person and still they see no fault of this.