Copied and pasted from Moneyfacts:
How to cancel a direct debit
You can cancel a direct debit at any time. However, make sure you inform the person or company that receives the payment before you do, as you could incur fees or penalties for non-payment of a bill.
This could even go onto your credit file and affect your credit rating if you don't pay a bill in a certain time frame.
Some companies (eg energy companies) offer discounts for payment by DD so you may lose the benefit if a payment is cancelled. Also if you use DD to pay your credit card balance you may get card charges