According to this you have a statutary 10 days cooling off period - you'll need to notify them by phone, and recorded letter:
Moneysupermarket.com - Orange Broadband: 14 Day Cancellation Period? Please Help!
Another posts says you need to cancel before it goes live.
edit - AOL give a 7 day cooling off period and help on how to cancel here:
How to cancel your broadband membership if you are within your cooling off period - AOL Help