No service from Discovery card and ******ion on interest rates.

Discovery card wanted me to renew my card after registering with their Bank. I told them I was not happy with the 17.5% interest that they ripping me off. Its 10% higher than my other credit card. They said I could request this amendment online once registered on my mobile. That does not appear to be the case. I have called 3 times and cant speak to anyone who can assist. I have sent 5 emails with no reply. I just want to cancel my card now. I even told them not to bother delivering my new card, but it was sent anyway. Their statement does not calculate correctly either, if overcharges R450 with no reference to any transaction showing this amount.