Defective Mazda CX5

Im not happy with the service of Mazda South Africa. In november 2016 i bought a Mazda CX5 from one of their dealers in june 2017 there was smoke and a grinding noise coming from the right rear wheel i took the car back to the dealership and they said it was caused by dust. in july the same problem occured on the right rear wheel and the dealership said it was caused by premature wearing of the brakes. Both rear brakes were replaced in August 2017. Now in October 2017 hardly two months later there is a grinding noise which is worse than the one before on the same rear braked because of premature wearing again. Mazda SA and does not see anything wrong with the fact that the car is causing premature wearing of the brakes they are refusing to replace the car.