pg has commented on this on HN before, the basic gist is that HN is like a newsgroup, you've submitted your comments in the public and just like Usenet, they are there to stay. Separate to that, if you were to delete your account and its comments, it would leave existing threads confusing and incoherent.
Do it like how Reddit does it, simply replace instances of $username with [deleted].
This is one way to do it, but it still creates stark discontinuities in discussion threads.
But you can delete threads, right? I frequently see '[deleted]' with replies to the deleted thread.