Mtn sucks

Despite their obligation/confirmation/sssurances, that they will cancel my account, after number requests (from end February to date) once I settled the account in full and send them proof of payment (which I sent to their customer care; retentions dept; and their legal dept on 7 January, I HAVE STILL NOT HEARD FROM ANY OF THEM. WHAT DO I HAVE TO DO TO GET THEM TO CANCEL MY ACCOUNT. I CANNOT JUST WALK INTO AN MTN STORE AS I NO LONGER LIVE IN S.A.