Charged for the services that I am not using

My bill has almost doubled and keeps going up. My four contracts have been blocked many times and I am hardly using it. when they blocked the main contract, they also block the Pay as you go contract that cannot make the limit to be exceeded. Last month I bought new SIM's and didn't use any of the contracts. They billed me R3000+ and they blocked all contract for exceeding the limit while these SIM cards of these contracts are not even in the phones. I asked today for the forth time, can they investigating what is happening, specifically who is using my account because someone is or MTN system is not billing correctly. I was told that I need to call them back on the 20th after the invoice has run. What does the invoice run has to do with checking who has been using my account? Why block pay as you go SIM cards, if the main one has exceeded the limit? Before my upgrade last year, people who work for/with MTN stole phones as if I was the one upgrading. I had to go to police station to make an afferdavit to say that people de*****ed me/MTN, even though they know and have proof to the fact and have the information of where and when it happened. I have not received my money back.