Error Debit

I cancelled my cell c smart data contract in November as I am emigrating from South Africa. I was told that everything was in order and that the contract would end on 5 December 2015. However, I am still being debited the monthly payment for a cancelled account! Over and above that the sim card is no longer valid, which shows that the contract had indeed been cancelled - I expect a full refund for the month of December! This is completely unacceptable!