Mtn does not keep promises made

Mtn billed me in June double the amount they normally do debit. I called in immediately and enquired and I was informed that my contract has ended but because i didn't cancel it, I lost out on some discounts. I instructed them to cancel that contract and even sent them an Email on the 1st that I want it to be cancelled. Ive been in contact with their Retention department, billing department and even sent an email to their complaints department but I just received a ref. number but I have been assisted. I just called the call centre and I was just informed that the cancellation was done after the 12th. So according to them I will still be billed double the amount which is unfair. I regret ever taking a contract with Mtn