Billed for Cancelled service

Here we go again, Thanks MTN I did receive the credit but again this month I am being billed for the Self Service Bundle wich was cancelled 20 Aug 2018.. How is it possible for MTN not able to remove this from my Acct. Now it is 4 months later, 7 different agents, 6 different ref's and still ongoing. Again credit will need to be arranged. MTN you responded by saying that you will notify me when the service is removed, STILL WAITING, STILL NOT REMOVED, how is this possible ??