Contract Cancellation

I had a 2 year contract with MTN, since Feb 2013, the contract had expired. I notified MTN on Thu 16/04/15 12:15 PM to cancel the contract after noticing that they have been debiting for data services I have not agreed too. I tried numerous times to call there call center to cancel but there is no response from there call center agents, appalling service, frustrating and time wasting. On the 17th April, I notified them that I had stopped all my debit orders, despite doing this, they continued to bill me each month, every month i had to go into the bank and have these transactions reverse which was being charged to my account. I sent MTN another notification for cancellation on the 27th may, and they continued to debit my *** June I sent them another notification no response and no cancellation. Yesterday I received a notification from MTN threatening to hand me over to there lawyers if i don't pay them the R81.96 owe them. They have a nerve to send me a letter, they don't want to accept my calls and they refuse to accept my letter on cancellation, Sorry MTN i will not pay the R81.96, you have been billing me for something i didn't consent too.