*********** staff made a mistake now I have to pay

I took out a WiFi contract at Cell C for R99 per month. They never put a cap on the contract I went back to the store and they said they would cap it. the next month I got a big bill again. I went back to the store they said sorry they will try again then it took 4 of them to finally put the cap on the commuter. but then I had a bill of R3196.71 that I can't pay and I feel it is not my fault because it is not my fault they never put the block on and I didn't know that. the manager says he cant help me. they wont accept any responsibility.