Best answer by Hikari
I am pretty sure that you would need to contact ID support team and ask them to cancel it for you.
You can contact using this Page
Personally (it just my opinion and suggestion) I don't think is wise to cancel direct debit because you may forgot to pay your bill one month due to busy schedule or different unforseen event and you might run into problems that you would prefer avoid. I would suggest to change a date of your direct debit to the one that suite you better, you can do it by contact ID Mobile or moving to pay as you go if you don't mind getting new number.