Vodacom refusing to cancel an already expired contract

I have been trying to cancel my contract with vodacom for the past three months (my contact expired in August already). Every time I call to cancel I get promised that a cancelation sms will be sent to my number and eventually when I see there is no sms sent and call to enquire they tell me it will only be sent month end. Come month end, no sms has been sent and by that time it's already late to cancel for the new month, and so the cycle goes... I pay my account with a debit order so I have at some time asked to change it so I pay in store and that way I will just stop paying but apparently that will just ruin my credit record so basically for the past three months I have been on contract with vodacom against my will. What am I supposed to do to cancel this contract?!!!!