MTN forcing me to go on month to month because I owe them

I am Very upset with Mtn, my Modem Data has come to an end however they say they will not cancel it since I owe them for the previous month, i do not want to go on a month to month plan, I want to cancel the contract, I will make a payment month end but I do not want them to keep my contract I was paying R500 plus they are forcing me to go on a month to month of paying R1400 for a data I no longer need. I do not want mtn anymore please understand, all I am saying is please do not send me anymore data, rather block my line for the time I have not paid the amount owing.