Debit order not stopped after stopping contract.

My contract expired and I stopped it in August this year. The last debit order was supposed to be at the end of August. I was debited again at the end of September and I reversed the debit order. I received another for invoice for October and did not make much of it as I thought that my contract had been cancelled an converted to pre-paid as I had requested. There was no debit at the end of October but I was suddenly debited on 13 November for R431, which is very inconvenient. I have been trying to call with no success.