Okay, after thinking a few minutes about the Batman-Joker/where do you put Dark Wizards if you’re determined not to use Dementors anymore problem...
Kill them. With great power comes great getting-held-responsible-if-necessary.
Unbreakable Vow anyone? Just give Dark Wizards the option “either you take an Unbreakable Vow to never knowingly kill/torture/Imperio a human being ever again, nor to ever knowingly assist in such, or we just execute you right now”.
Oh, no, this is much better. Magical evilness castration.
Kill them. With great power comes great getting-held-responsible-if-necessary.
Oh, no, this is much better. Magical evilness castration.