CANCEL THE CONTRACT PERIOD!

I have been trying to cancel my contract without any luck. I keep paying for the service I no longer need all because MTN won't allow me to cancel it. The MTN store at woodmead, the consultants told me they can't cancel the contract. Was advised to call 135, however somehow my phone doesn't call that number, I don't know why, but it's not. The consultant said they also can't help me with that. Been sending emails to customer care and retention team, none is assisting. My request is simple CANCEL THE CONTRACT PERIOD!