wrongly billed

I have had the worst experience with telkom. so I signed a contract and I indicated that I dont need any extras and I put the spent limit at zero to my surprise after 5 months I get a bill with an extra R500 when I ask how is that possible as I rely on the fact that if my wifi finishes it is finished. when I call in am told tge system makes a default limit after sometime. so which means telkom is making us invalid contracts as their system dictates for us...this is **************** and ********* how can a contract hold no water over a system. am so very disappointed at telkom and their business ethic