You email PG and ask him politely to delete them (there's no way to do so built into HN - though comments/submissions can be deleted or modified for an hour after being posted). However, I've never heard how well that works. Given that someone asks the question fairly frequently, and I've never seen someone complain about PG refusing to delete their account, I would guess that it works fairly well, though.
A previous Ask HN:, regarding deleting the account.<p><a href="http://news.ycombinator.com/item?id=2088545" rel="nofollow">http://news.ycombinator.com/item?id=2088545</a>