I don't think that would work today, but what we have today doesn't work either.
Fence in a few hundred thousand acres of wildlands.
Send sentenced criminals in with a minimal backpack of survival gear, a waterproof survival manual (including a minified calendar), and their exit date and fingerprints engraved on a piece of metal (maybe their firestarter).
You showed repeatedly you didn't want civilization, so here - you get what you chose.
Man the walls with military or national guard troops who rotate out regularly (makes it hard to corrupt them) - come within 500 yards of the wall (interior or exterior) anywhere but a defined ingress point and you'll be shot after due warning.
Approach an egress point from the interior with any other people and you all get shot. If you can't show that it's your exit date promptly on reaching the egress point, you'll be given five minutes to get clear or be shot.
I'm no longer as convinced it's a good idea, and I'm sure the logistics are massively harder than I'm making them sound. It's certainly harsh and merciless.
Still - there is a certain elegance to it.
It is a very Heinlein plot device, now that I think of it.
I think the concept works much better if it's wilderness, though. Even a ruined, abandoned city has an endless supply of the artifacts of civilization, so those imprisoned don't really get to understand what it is they've chosen.