Request to cancel debit order

I have on two occasions sent emails to Cell C requesting that they stop debiting my bank account as I would be making direct payments to them. I have not received any response from my emails beside a constant reminder to ensure that there are funds for them to debit against.<br> <br> When phoning their customer care centre the only thing they are interested in is doing their security checks by asking your ID number. This places one at risk. They do not want a date of birth or ask any other security question. All they are interested in is your ID. <br> <br> How can I cancel this debit order. Please help.