MTN Yellow Bucks rewards programme ****

I am so disappointed with the MTN rewards programme called "Yellow Bucks" which are given to you for rewarding you for usage of Airtime and data on the MTN network. Yellow Bucks are given in vouchers but Medirite vouchers are not valid and Medirite say that they never entered into and agreement with MTN. I go to Checkers to use my R50 voucher there and they say at five stores its invalid. I'm sitting with a bunch of vouchers totalling R500 and they can't be used anywhere and some expired. What a ****. I reported the matter several times and they have yet to come back to me in the two months of reporting the matter. Shocking service and ***** practiced by the Mobile Giant MTN