You cannot seem to delete your account.
However, you can contact them easily: http://www.smartphowned.com/about – ask them to do it manually for you. Make sure to include your account details in the message body.
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.