Having been an admin on another forum, deleting an account is the worst thing to do, as it removes all the posts the user (you) made, which can really screw with threads, especially ones you started and/or posted a lot to.
What I do if I want to permanently leave a forum/site is remove any personal information there (signatures, "About", etc) then change the email to something random (either myself or asking for mod/admin help) and then changing the password to something completely random.