Unable to cancel contract

My elderly father is trying to cancel his contract with Telkom. He have notice on ********** 0 with reference number ********** 9. To date, it has not been cancelled and some else is using the SIM card at our expense? Asked Telkom to block the SIM but can’t locate the SIM? Yet, I just spoke to the guy using the SIM who is refusing to stop using it. Why Telkom is the contract not canceled as yet?