Debit order NOT being cancelled

I moved to the UK in Feb 2015. Since March 2015 I have been corresponding with MTN customer services to cancel my debit order of my internet that I had with them. I have been on the same reference number since that time. Two of the employees CONFIRMED that my request have now been dealt with (one in around July other in September) and that the debit order will now be cancelled. But the debit order have been going off every single month since then. I have worked with my bank to reverse these debit orders as I am not even SA and definitely not using any of the internet services. Since MTN keeps changing the reference nr, my bank is unable to block the debit order. Since their customer services takes MONTHS to respond, and then they say the problem have been fixed but is has NOT, I am not even trying to log another complaint with them. Absolutely horrible service!