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...

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".

I can think up of possible ways out of this meta-problem, in order to sustain the dilemma: Perhaps really powerful Dark Wizards require too vast a portion of magical power to sustain the vow. Perhaps there are dark rituals whereby using them, Dark Wizards can break out of even an (ill-named) Unbreakable Vow. Perhaps Dark Wizards tend to have made other rituals that already make them immune to Unbreakable Vows... Perhaps unbreakable vows need be really really specific in some weird manner like "I will not kill Bill Weasley", and "I will not kill Charlie Weasley" necessarily are two separate vows, so that "I will not kill any human" isn't enforceable...

But these are additional problems that are not yet mentioned/listed/foreshadowed in the story. Ugh, Unbreakable Vows seem something of a game breaker right now.

Sidenote: Whenever I think of something such, I worry that the author will think he'll have to rewrite/revise everything he had already planned, and that we'll never get an update again. Not my intention, I swear.