Investec - unable to reverse Debit Order !

Investec bank do not have the option on their app to review and block debit orders! They reversed the 1st MTN *****ulent debit order but not the next 2 months. MTN SP has a *****ulent debit order against my account since 3 months ago. I did not approve it / have never had anything to do with MTN. Investec say I must contact MTN to stop it - I am currently not In SA for several months. My wife went into MTN at Tygervalley, Bellville; and were told I have to come in person! I just get a generic computer reply to my email with and MTN ref number. Very frustrated - I have no control over my Investec account being used by ******** elements within MTN!