Contact customer care: http://shutterfly-2.custhelp.com/app/ask and ask to have your account closed. If they ask for a reason just tell them you want to close the account.
URLS:
http://www.shutterfly.com
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.