MTN overcharging me on a contract with a limit, adjusting limit without my consent.

I have a contract with MTN business. The contract states that there is a limit of R350, however MTN decides to open the account and allow me a limit of R10 000. I have had to request that the original agreement be upheld atleast 3 times in 2018. I only realize that without my consent the account was changed when I see exorbitant amounts being debited from my account. I expect the phone not to allow me to connect to the internet or make calls when the agreed limit has been reached. In September 2018 I paid R1722, in October R3850 was again debited from my account and again R3751 in January 2019. All this time my limit was capped at R350.