FAULTY DECODER REPLACED WITH ANOTHER FAULTY SECOND HAND ONE !!!!

I took my decoder in because it was faulty. They exchanged it with a second hand one although I have insurance on it . I was also not told that it was second hand. Two weeks later the replaced decoder is giving me exactly the same problem as if I have time to go to multichoice every second week !!! They don't even have a complaints department . I was told I can only send an email ,but they don't even respond to your emails !!! I'm really disgusted in the service from multichoice /DSTV!!!