VODACOM SENDING AIRTIME ON EXPIRED CONTRACT

I settled my cellphone account in June of this year, but vodacom didn't cancel the contract as they sent me airtime for july and August, in which both months i sent further cancellation letters and paid for the airtime, today three months letter they send me an sms stating that i owe R1007.00 and that they will disconnect me!!!!! I purchased the phone on a 24month contract which expired in June 2015....Its unfair that i have to continue paying them for failing to cancel my contract , for failing to do their jobs. As a consumer how can i protect myself from such service providers?<br> <br>