I assume you mean cancel your service rather than direct debit. You can do the latter through your bank anytime you want but it will almost certainly cause a heap of problems. There isn't an online facility to cancel so you have to give your 30 days notice over the phone or in writing. I'd recommend recording any conversation you have.
Hi jimmy691, the direct debit can be cancelled directly with the bank as this is the agreement between yourself and the bank to pay us. Please be aware that if the direct is cancelled this will not stop bills being produced and you would have to pay via an alternative way if the direct debit is not reset up - Chris