Inhumane treatment from Vodacom

A month before expiry of my vodacom contract, I wrote an email to vodacom to notify them of my end of contract. They did not cancel it, instead they said l must call their customer service. I then called and they asked me a number of questions and clearly, I couldn't answer all of them correctly, they then said that they will not cancel it because I couldn't answer those questions. They continued to debit my account until I stopped it at my banking institution. As it is, I'm now blacklisted in addition to this ill-treatment. I'm so aggrieved because I did not give vodacom permission to give this inhumane treatment