Mtn is robbing me!

I signed up to a contract a few months ago. I thought it was a top up contract. I no longer use the account that I have my debit orders come off so I just keep enough money in it for my debit orders. My mtn debit order came off unpaid and I was told it was over R1000 because I'm not on a top up contact. I paid and made my contract top up. Next month, same thing happens and I am billed over R1000! They claim this is an old amount and assured me that it would go back to the R300 and something the following month. I had extra money in the account this month and R1000 has AGAIN been taken off by Mtn who once again claim this is an old account. I kept asking them how many big amounts do I still owe so that we can prepare and each month, they said none! I am again told that next month will be the low amount but I cannot afford these unprepared huge amounts of money! To top this off, I have spent fortunes on buying airtime and data! I cannot stand their horrible customer service, lack of communication and the fact that they are charging me fortunes, without making me aware of it in advance. I just want to cancel my contract and move to a different network where I will get better service!!