take a top up contract with a limit but no limit

I would not get a mtn contract ever in my life again i went and sign up for a contract that cost me R499 pm then after your package airtime and free minutes it wil not make any more calls and you can top up by buying airtime at any garage or shop. So we used the phone it never cuted out and my first debut order was R1200 i then went back to mtn festival mall where i got the contract. The sales person Nico said he would do i query and told me they will pay back the money that was a slip up on mtn side 30 days later still no refund so i went to the bank and reversed the debut order. Today was time for the debut order to go down and again the took just over R1000 i went to mtn festival mall again then Nico told me no mtn has fixed the problem my phone will have a limit but i must still pay the R1200 of the privious month. They have suspended my services now today. I just dont agree with mtn it isint my fault that ther was a problem on their side why must i pay that money. I know i have used the phone because Nico told me use the phone until you cant make anymore calls and then top up. What should i do now