Do not want to refund a bad Black Berry 9720 - 2nd

We bought a phone Black Berry from them. This phone is not working properly, actually the worst phone I ever had. I complained and asked for refund. They refused, send it in for repairs, could not fix it and replace it with the same type of phone. Now I experience same problem again, ask for refund again, they refuse want to send the new one in for repairs. Once again I must go to them and wait again for the phone I do not want, why can't they just refund it. I told them last - it is a piece of junk, again this time. Why do they want to force this on me as I clearly do not want it any more