Be logged in on the site, then use the deletion link. A full termination that is, good work.
URLS:
http://todoist.com/
http://todoist.com/Users/showDeleteAccount
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.