Earlier quoted context omitted.
He was correct to do this, because it was funny. Any true longtime forum poster would recognize this. (Conversely, if it wasn't funny, he should've done it differently.) Although doing Lowtax things may lead to turning into Lowtax, which is probably bad. I can certainly tell you it's not a serious problem, because it'd be illegal if it was. All serious problems with websites are revenue issues or legal issues.
It's not funny when the victim isn't in on the joke and is distressed by it. (Sure, the victim was (probably) a highly unsympathetic and stupid person. What justification is that? It's schoolyard-bully sociology to attack unsympathetic weirdos. Most bullies grow out of it and live with regrets. But 'spez is a fucking adult! What's he doing lurking in the worst subreddits of (his own) website, searching for prey to ga…
Basically, it's PvP. This is pretty obvious from the design; you post and people reply to you, which they mostly use to argue with you. An example of a product designed for people to be nice to each other is Discourse, which isn't much like HN or reddit.
People do have human rights, which is why I said what matters is if it's illegal or not. Putting slanderous posts under their name would be a real issue - if you think this happened go ahead and sue him - but the most common one would be doxxing: posting your IP and personal info from the admin console. Which he's probably got access to.