Account still billed even when cancelled

I have cancelled my sim contract with Telkom on the 30th of August and i was told that it has been cancelled after i paid the money i owed them,now i receive a statement saying i need to pay additional money.I told the consultant that i am no longer in possesion of the modem nor the sim card, and i asked for it to blocked,How do i pay for something i have cancelled. Acc no:***