Excessive billing on account

I have had 3 contracts with cell c. The value of which was suppose to be about R650 per month on a fixed amount. In the month of January over R1800 was deducted from my account. When I inquired the customers service consultant on the 140 number said that it is because my son's cell was set as a open line. My son is a student if I sent my husband and my phone for limited usage there is no way that I would have given him an open line. I have since cancelled my contract but I am unable to cancel the other 2 as they are still in contract so I tried to change my son's contract to minimise the usage. The consultant now tells me that i need to pay an additonal R1200 and that my son will still be able to purchase data without my consent. Surely this should be against the credit act. No one from cell call me to advise me on the additional usage. they just allowed the purchase and deducted as a debit order. If my son purchases over R10 000 this means that it would be allowed yet he is unemployed. Cell c should not allow a third party to use my bank account as they feel fit. I demand a refund. Cell c should not be able to afford clients unlimited credit limits to purchase data.