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.