I’ve heard ideas that prisons could be voluntary in a completely free society. First, a lot of people who have gone to prison for victimless crimes (using and/or selling drugs for instance) would no longer go to prison. Second, people who have committed violent crimes may choose to go to prison to “work off their debt” and show willingness to atone for their crimes. One could imagine that many employers, businesses and people generally would ostracize a person that would not be willing to go to prison, but on the other hand would be more willing to again accept that individual if he did go to prison.