Still overcharging me for device payments

This complaint is been on going since February. They are overcharging me for a device that I have paid off in December 2014. They keep on telling me that they will refund me. I've waited for 2 month and realised that the will never refund me even after I have send them proof that my device was paid off. The only other thing I could do was to cancel my contract, which I've done but they KEEP ON CHARGING ME FOR THIS DEVICE\ THIS COMPANY IS R E T A R T E D!!! BUNCH OF T H I E V E S. Do not sign up with them