High Bill 200% more than the agreed amount

I am so unhappy with the high bills that Ive been receiving from the account mentioned on the subject line. Initially when I took out the account I was hoping I will just pay R250 but never a day that occurred. Although I had had stated in the beginning that my spend limit must be Zero but seemingly my request was disregarded.

I want to know what would be the procedure of on cancelling the contract.