In the FAQ you can read this is only possible by PM’ing an admin or e-mailing the site with your request:
info [at] instructables.com
See the FAQ question: http://www.instructables.com/community/how-to-delete-instructable-account/
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.