Telkom doesn't want to cancel my contract!

I have had my Telkom mobile contract for over 24 months (mandatory period is 24 months). I have tried countless times (via telephone, email and in-store) to cancel my contract, but I have received no response in this regard. Since my contract ended, my bill has doubled, even though my usage has decreased. I simple cannot afford this hefty price increase and need to cancel my contract immediately. I don't know what else to do to get assistance!