As for prisons, solitary is an option, however one could argue that it's even more inhumane then capital punishment.
The prison one is one that never gets enough focus, mainly because we have an attitude of writing off people that go to jail and that whatever happens to them there a rough form of justice. But then it's not really great to have people sentenced to life in prison without a chance of parole now performing this kind of "justice" to other people that might have a chance to reform and go back into society.
Just as a single example, Freddy Geas was a mafia hitman who was sentenced to life in prison. He killed Whitey Bulger, another murderer. Sure, Whitey wasn't innocent, but did he deserve to be beat to death by another murderer under state care?