They're already being punished.
In the story, society has matured to the point that villainy of any kind just doesn't happen; nobody thinks to do it. Like most post-scarcity "space communist" societies, your society is a bunch of actual pacifists.
Even if somebody is hurting others, that doesn't give society the right to hurt them in turn. Turnabout is not fair play.
Faced with this truly horrendous criminal, they are at a loss as to how to punish them. After deliberating for several months, they come to the perfect solution:
They'll leave him alone.
He will not be punished in any way, and will instead be left to his own devices. Everybody will leave him alone so long as he doesn't physically harm anybody. In fact, he will be left completely alone. Nobody is to react to him in any way, shape or form.
If he starts experiencing boredom, depression, and soul-crushing anxiety, that's his problem.
“[The tyranny of the majority] ignores the body and goes straight for the soul. The master no longer says: You will think as I do or die. He says: You are free not to think as I do. You may keep your life, your property, and everything else. But from this day forth you shall be as a stranger among us. You will retain your civic privileges, but they will be of no use to you. For if you seek the votes of your fellow citizens, they will withhold them, and if you seek only their esteem, they will feign to refuse even that. You will remain among men, but you will forfeit your rights to humanity. When you approach your fellow creatures, they will shun you as one who is impure. And even those who believe in your innocence will abandon you, lest they, too, be shunned in turn. Go in peace, I will not take your life, but the life I leave you with is worse than death.”
- Alexis de Tocqueville