Broken contract phones

I took a contract few months ago BB Z 10, had the phone for less than a month and had send it back for repairs as it stopped working, this happened 3 times since I hv this phone, I got the phone back from them a week ago and again its giving problems. They debit every month for something I can't even use properly. I had to buy a second phn in the meantime. I called them today and again they want to collect the phn for repairs, and I'll probably get it back in 2 to 3 weeks time not using it again for this month thn they want to debit again!! Hell no!!! I demand that they cancel the contract pay back my money I paid so far and collect they broken phone. I'm done with Cell C.