What, no mention of life in prison, which is what some criminals are doing already, like Josef Fritzl? I agree with this option, too, because some people are just too incorrigible and too dangerous to be around peaceful citizens. I doubt that a guy like Fritzl would suddenly have a change of heart after imprisoning and torturing his daughter for 24 years.
In a sense, life in prison is harsher than the death penalty because while a prisoner's life isn't threatened, they have to deal with the fact that they'd spend the rest of their lives with no freedom since the death penalty is an easy way out. It's not just punishing them; it's protecting others from their evil.
Slightly off-topic: I've been in a similar situation with people who are not criminals, but are toxic in other ways. I've given them chances, but they took advantage of my kindness. I realized that the only way to put a stop to their behavior was to distance myself from them. Their "punishment" is not having me in their lives again, though it's more about self-protection than revenge.