All these features are covered in the FAQ What does [deleted] mean?, Can I change my username?, Can I delete my account? https://news.ycombinator.com/newsfaq.html
We try not to delete posts that got replies, because doing so would be unfair to the other commenters in the thread. What I've done so far is reassign it to a random user ID, so it's as if you'd used a throwaway account to post it and there's no link to your main account. Does that work?