Debit order deduction after cancellation

I contacted Cell C during the month of October. I was due for an upgrade but decided to cancel my contract. I was told to call them 1 Nov 2017 to give notice and the cancellation would then be effective 30 Nov 2017 (last debit order to be deducted end November). I received a SMS after the call confirming all the information. I now found that my account was debited at the end of December again! This means I am paying for a service I no longer use - it's unacceptable!!