*******ly debiting my account

I had a contract with telkom which came to an end last year july/August. I called them to cancel it and they said I was late for the current month and I said that the must cancel for the following month then, they did not. They continued deducting money from my account. They are still trying to debit money from my account - what permission does telkom have to debit money from my account when I do not have an account with them?