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.