MTN over -debitting me since February and cutting off my internet

I have a cell c contract because I wanted to have a dual sim phone. I am however a loyal mtn clirnt for over 20 years. I then took out a modem data contract and wanted to cancel my month to month cell contract to pay as you go. I was told that I have to call the retention department to do so. After waiting for over 25 minutes I was told that I need not cancel they will still give me data for my phone but ii will have a zero balance contract and will only be charged the R399 for the modem contract. This happened in February this year. Since then they have still been deducting between R600 to R800 per month. Everytime I have to call the retention department and then after waiting a minimum of 25 minutes get promised that they will correct the matter and eventually they acknowledged that they must rermburse me. I eventually ended up canceling the Debit and informing them that they must cancel the zero balance cellphone contract and only deduct R399. On the 17th September the again deducted over R800 which I then tried to block and reverse but I have yet to receive proof that the money had been returned. I also had already sent a complaint email to the complaints department but received no response. This week I received an sms stating that If i do not pay R600 plus immediately then my services will be cut and I will be handed over to the credit bereau. Since yesterday my internet has been cut off. When I called I was told that I must get the retention department to credit the money they owe me to their finance department so that they can reactivate my internet. I refused to do so as i have contacted them several times and always am kept waiting min 25 minutes before I the call gets answered. They then committed to emailing the retention dept to contact me which up till now has still not happened. I am extremely frustrated and dissapointed with MTN. Loyal support for 20 years and this is the service and treatment I get.