MTN Contract Cancellation loose all data and airtime but you must still pay..*******.

I would like to cancel my contract and MTN make it very difficult, my 24 month period has come to an end and due to poor service from MTN I have chosen to cancel my contact. apparently whether I cancel or go onto prepaid I will loose my airtime and data I have paid for and I will still be billed at the end of the month for data and airtime I have not received. this is theft and steeling from the consumer. How can MTN take away my data I have paid for and then have the cheek to charge me again for the contract when all my airtime and data is depleted??? and have cancelled..