Excessive data charges even after cap requested

In Sept our company rec a very high data bill of over R5k for a contract that is normally R100 a month. On querying we were told it was all legit and that the next month had already racked up 4k and was increasing. I immediately requested a cap IN WRITING on the data contract but was hit this month with a debit of R16K. Since requesting the cap, the offending data sim and ****le have been locked in a drawer and not used but all the excess usage was after this date according to the itemised bill. When calling to query, I was told they would \investigate"and while the operator could not see if a cap was in place as requested"