From https://news.ycombinator.com/newsfaq.html

"Can I change my username?

Yes. Email hn@ycombinator.com and we'll help.

Can I delete my account?

We try not to delete entire account histories because that would gut the threads the account had participated in and is unfair to commenters who replied. However, we care about protecting individual users and take care of privacy requests every day, so if we can help, please email hn@ycombinator.com. We don't want anyone to get in trouble from anything they posted to HN."

I remember reading at some point that if you email dang requesting a deletion he’ll process it. Not sure if that’s still true.