MTN failure to cancel debit order

Some months ago I cancelled my existing package and signed up for a new one at a significantly lower price. This is contract is active and working fine. The problem is that my wife's ABSA account is still being debited with R508.00 every month for my cancelled contract. Both she and I have made numerous calls to MTN (5 in total) to rectify the situation. Nothing has changed. Once again today her account was debited with R508.00 and ABSA say they are only permitted to cancel debit orders under R500.00 - a ridculous situation. The paymentsd aren't lost - they are credited to my new contract which is only R129.00 per month so there aleady is a sizeable credit built up - well over R1 000.00