MTN useless contract

I took out a contract at MTN in July 2017, and specifically told the lady assisting me to put a limit on my contract to R1. I looked at my bill for the past 3 months, and MTN deducted an extra R250 per month. When this happened the first time, I went to the shops and ask them to correct this, and they assisted me, just for this to happen again the following month, and the month after that. What is the point of setting a limit when you take out a contract? Is this MTN's way of "*****ing" money? And when you do go in, the assistants can't really help you, all they say is, "we don't know what happend". Then they will try to put the onus on the customer saying it was changed via the phone. I mainly use the phone just for the minutes and not the data. How many times will this happen still? And I'm sure I'm not the only one with this problem.