From their help forum:
You can downgrade to a free account from the ‘Plans and Pricing’ page, which will cancel your billing.
To truly delete your account, send your request to: firstname.lastname@example.org.
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.