From http://jacquesmattheij.com/The+Unofficial+HN+FAQ:

"HN does not have a way to delete your account directly, but you can ‘anonimize’ your profile by removing any identifying information there. The email address is not visible for outsiders so you don’t need to worry about that, simply wipe your about box and call it a day. If you used your real life name as your username it’s a bit more of a problem, especially if your name is not ‘John Smith’, you could try emailing the owner of the site but I would reserve that to those occasions where you really think it matters.

If this is to ‘unsay’ stuff that you wrote in the past then that’s a good reminder to think twice before you hit that submit button lest you cause someone needless work."