> The best school to become a thief is jail.

I think that's a US thing. Other countries seem to be genuinely more rehabilitative. Looks like 55% of the state prison population is there for violent offenses: https://www.prisonpolicy.org/reports/pie2020.html

Handling non-violent offenders away from the violent ones might be a good start.

I completely agree with this. You will find a ton of stories about nonviolent people who had to become/join violent groups in prison just to stay alive and not be abused incessantly.

You can't you just have to let it happen until they implode and you can throw them in prison but at least you've limited their recruitment.