From their site:
Sign in to your Admin Console. Make sure you are signed in as the owner, as that email address is the only one with upgrade/downgrade privileges.
Click on Settings in the Admin Header.
Choose General from the drop down menu.
Scroll down to Billing & Invoices. There’s a link that says: Delete my account. If you’d like to delete your account (which will get rid of all your data and forums), click this.
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.