You cannot delete your account.
Please contact them via http://support.iconfinder.com/customer/portal/emails/new?q=delete%20my%20account
In the message body, ask them to delete your account, and supply your account details.
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.