Over deductions

I have a contract with Telkom, the amount I’m supposed to pay each month is R570 however since I took the contract they have been debiting more than the agreed amount, I have called numerous times to enquire about the extra money they have been debiting, I was told that my spend limit was not set to 0.00 which I advised them that I was told it was in 0.00 I further asked them to set it to 0.00 which they said they did but to my surprise they will debit more money again, I can’t even set it on my side coz it does not give me the option for 0.00… Telkom is worse service provider, I wish I can cancel it