Billed for Contract which was ended

I had cancelled my contract which was effective 4th Jan 2018. I am on a top up package and accordingly I was billed in December 2017 for the month and inclusive of the 4 days for January 2018. My number thereafter became prepaid (not on the top up contract) and I subsequently ported to MTN (a much better network). Today, I notice that Cell C has debited my bank account for January 2018 for the full monthly premium. My contract is terminated and therefore this debit order from Cell C is ******** and has left me strapped for funds in my bank account. This is ridiculous and unacceptable. Please do a refund effectively back into my account and ensure that funds reflect immediately. I do not care about your administration issues as Cell C is full of issues. This has also resulted in unnecessary bank charges and I feel Cell C should pay up.