I’m not a fan of authoritarianism. It does train kids to think about the consequences of what they do, but only in terms of if they are going to get caught and punished, i.e. what are the consequences FOR THEM.
Far better imo to train kids to think about the consequences for other people.
Ton’s approach is the best mentioned so far.
Why is bullying bad? Because it upsets other people, and this is bad. So why not make that clear to the kid concerned?
Discipline has a place sometimes – when your kid is just trying it on for example.
I did a spot of bullying when I was a kid, in a single incident. It was inadvertent though. The other kids started a rumour that this kid thought he could beat me up, and being a competitive type I challenged him. It never occurred to me that this was anything other than just normal schoolground stuff until I found out he had been to scared to come to school the following day. Once it was made clear to me what I’d actually done I was horrified.