Unauthorised billing

I took a contract with Heita mobile in around 2012. The contract payment agreement per month was approx R100. I haven't been using the sim card for a while now , but surprisingly Heita has been deducting abnormal huge amounts from my account without any approval of increasing the monthly limit. I asked for a statement from March and discovered that they have been deducting amounts in access of R900 for a while now. In Sept was R2000. I contacted the Heita on 20/11/2015, ref no.***6 after noticing this, I requested a statement to see what was happening, since i haven't been using the number for more than a year now. I discovered that the limit was changed without my concern and there is someone who has been transferring airtime every month from my number to other Heita mobile numbers. Heita refused to tell me who the numbers belongs and told me they will open a ***** case and will give me feedback in the next 2 - 72 hours. To date they haven't called. I called again on 04/12 and still failed to suspend the service. Surely this is ongoing since the sim is active. I want to be refunded all the money they have been deducting without my approval from the time this started.