I've had one banned for saying relatively tame things that made the mod defend himself with, "you don't seem to like the site very much." I then created another on the same machine, and it was silently banned before I even made a first post.

Just an anecdote to answer your question. This was about 2yrs ago, for the record.

i had my blog banned for outing Daniel B's identity in that laptop for techcrunch posts debacle.

it was also a silent ban, so I didn't even realize the site was banned until someone told me that the HN link I gave them didn't exist.

I was ghosted for making a somewhat aggressive post about H264 once. It was a moderator's mistake and super hard to get someone to talk to...

Look at the linked profiles and at the comments they posted. If you go back far enough, the sea of dead comments (turn showdead on) will stop and you will see comments with up- and downvotes as well as responses. That’s when they were banned, so the reason must be somewhere there.

I couldn’t find it, though. Seems arbitrary to me. Looks like commenters with slightly below average quality contributions who wouldn’t be missed by anyone were banned.

If you look at the comment history of a banned user, you can often see the very point at which they were banned (because comments posted before the ban aren't killed). It's usually just a single low-quality comment that triggers a ban.

I don't presume to speak for HN but the impression I get is that you can be banned for seeming disrespectful, either to the HN institution itself or to other users.