Cell C trying to rip me off

I recently cancelled my cellphone contract 2 months in advance and paid off the remainder. just to get a message saying they will debit my account for a similar amount 2 weeks after. I tried phoning and going to the store in Greenstone to deal with people who didn't know themselves or were just too lazy to try.

My number has already been ported and I've got the SMS for cancelling and the reference number when I did the EFT.

So I'm over dealing with Cell C, terrible customer service and lack of knowledge. problem is still withstanding.