Billed more than 3 times my actual account!

I have a contract with MTN in which I bought an Apple MacBook. Part of the deal was I received a MTN sim card and USB internet stick. I use the internet stick and sim card to access my account with Afrihost and have never been billed more than R700 in a month. My actual payments are R669 and only twice have I been charged more when I had used the MTN internet service past my limit. The sim card has never been inserted into a cellphone and for the last few months I've only ever accessed the internet through Afrihost which I pay separately for. In order to access the internet via my MTN account on my wifi I have to physically change settings on my wifi router, which I haven't. To add onto all of this I was out of the country for nearly two weeks in January, yet my January bill was over R1800. When I queried it I was told that I had accessed the internet to downloaded ringtones and pictures from my 'phone'. All of which is impossible. I've already asked my bank to reverse the payment and am cancelling my debit orders with MTN. Thank goodness the contract ends now in March!