Monthly overcharges

I got an iPhone 5S with MTN on contract in July 2014 and I signed a contract that I was going to pay R460 every month for 24 months. However, on the first of every month, MTN debits an amount of R676 from my account. I send them countless emails every month asking them for a refund. I was only refunded twice and now my emails are being ignored. All I get is an email with the following reference number [#***] and nothing is being done. Last month an amount of R575 was debited from my account and I was told that I have a usage limit of R100 which I didn't even sign up for and something called a packet switch data. MTN continues to rip it's customers off thinking that we won't notice the money being debited from our accounts. The network is VERY BAD, especially at night! If they continue to overcharge me, I am going to reverse those amounts every month.