Billed R1800 for a R160 contract with closed line

I received an SMS from my bank for a debit from MTN for an amount of R3163.95. I contacted MTN customer care and they advised me that I used data to the value of R1800 on my data contract. I then advised the consultant that I have a contract for 6GB with a closed line. My internet access should have ceased once my data was used up. The consultant was complaining about a bad line and that he couldn't hear me and the call was disconnected. I have 4 contracts with MTN for many years now and I never had trouble with their network but I always had trouble with their customer service. Last year 2 iphones were taken on my contracts and I had to take time off from work to go to the police station for an affidavit and take it through to MTN to get the transaction reversed. I can list many other problems that I had and now I cannot take time off again to get this sorted out.