Poor service when returning within 7 day cool off period

I have been struggling since last year October to cancel my contract with mtn during the 7 day cooling off period. The device was ordered online and delivered to me. Within 7 days I had received a better deal and returned the device to their greenstone store. After this I had contacted the call centre to try and cancel but if you're not using an mtn number they don't give you the same options. I've tried various other ways by posting on the customer service site on their website, I've tried calling customer service and I uave been into other stores and no one is able to help me... they keep on debating my account every month but I do not use their service. I would please like you to stop this and just give my money back its already like r3000 that you have taken from me MTN