Why not make prison more humane instead of abolishing it? Prosecute crimes against inmates, allow independent observers inside, etc. That would also take a lot of political courage, though.
And in the "crimes against inmates" it has to be pointed out that many of them are committed by the jailers themselves. Those crimes need to be punished with severe penalties.
Short of removing the executive branch's monopoly on criminal prosecution, I don't see how it can be done, and there's all kinds of problems doing that (for one thing, unless you also abandon protections against double jeopardy, you invite sham prosecutions to protect the guilty.)
