i want to cancel my contract

So after enforcing my CPA rights and on 2 separate occasions asking them to replace the faulty device with a new hands set and not repaired or refurbished unit, they chose to repair it and send a refurbished unit back that has terrible screen bleed. I was contacted by a consultant to have the phone booked in for the 4TH time! I don't have the time for this. I want my contract declared null and void and Vodacom to keep their ****py device. I should have just stuck with my iPhone. I want to cancel this contract at no cost to me except what I owe for the month. That our change my device to an iPhone which i will gladly pay the difference for.