cell C is reaping us off with incorrect debit orders.

Cell C it started when my data contract come to an end. I was paying around 300 buck for 30g day 30g night. Cell C service provider called me and tell me that my contract is coming to an end and discount will be removed but if I would like to go on with it with a discount paying just a bit over 300 Rands. I agreed. Amount was almost the same with current one. After 4 or 5 months when that old contract supposed to be ended, I've found that I was billed double of what I was supposed to be billed. Apparently someone made a mistake and discount was not activated. After calling them and after investigations. They found out that indeed they were on a wrong side. They said to me we can't refund you but next coming four months you'll be billed free. I agreed. Then things were great till now this January. Back to paying double amount again. When called a lady said discount was removed. By who? she doesn't know. After 7 months or so. The lady said she will escalate the matter. I feel like cell C must be stopped to do as they wish more especially in our bank accounts. If they feel like they needs to make some extra cash they just charge us extra money. Wish we can find a better solution for this. If they refund us they must start to pay us interest of what they took. They are getting away with "crime." Someone please help me!!!