When it comes to Stefan Molyneux's philosophy that you should never spank your kids, he does have a blindspot: some people are simply incapable of having any morality past "if I do X I will get punished." Stefan may have been able to discipline his own daughter without spanking, but not all people are going to ever reach the intellect of being reasoned with. Punishment is the only thing they understand.
When it comes to punishments for crime, the same approach is necessary. Do you think the vast majority of thieves, robbers, murderers, or other criminals of these types have high intellectual capacity? I would say no. Therefore, punishment must cater to this level of intellect, because a prison sentence or in the most extreme cases, the death penalty, is the only thing these criminals will be able to grasp.
Yes, the state's monopoly on legalized violence is a major issue, and it is a real concern with the Cathedral looking to enslave us. So I will offer another example of punishment without the state. If a good portion of the population is armed, they can deal with the punishment more personally. A thief will understand that being shot is a real risk for his actions, even if he realizes it far too late. He will not later on reason that his stealing was wrong, so harsh punishment it is.