But you could.
And you wouldn't even need a reason.
In fact, you could ban AND make the whole conversation disappear. And nobody would ever know.
That's a problem.
It’s not a problem, you are owned nothing by participating in an online forum. Your participation is a privilege, not a right. You are free to participate elsewhere.
HN moderates mostly transparently, which they do not have to do at all. That demonstrates respect for their participants, or ideals, your pick.
If he did this habitually, I think word would get around. This is a pretty small community.
> And nobody would ever know
Considering how quickly they make new accounts, I think you're underestimating banned HN users by quite a margin!