You can’t delete your account, unfortunately.
Try to ask for deletion on the forum, or ask for account removal using this form: http://letters.gardenweb.com/letters/nph-ind.cgi
If this doesn’t help, you can always anonymise your data. Read the tip below.
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.