MWeb - unauthorised, *****ulent debt order and no way of stopping it!

Before I left the country last year I spent many hours on the line with MWEB to ensure that my internet line would be cancelled. I was assured that this was done and that all would cease in the week of Jan. For the last few days MWeb has been trying to take money off my account via debt order. I've tried to report the ***** to my bank and get them to stop the debt order. My bank (FNB) says they can't stop the debt order because MWeb uses a difference reference for each debt order - WHAT? I have no line with MWeb anymore and still they're allowed to simply take money off my bank account with impunity. How are they legally allowed to keep doing this?