You could just strip their magic.
If there exists any ritual that happens to permanently remove a portion of somebody's magic (Unbreakable Vow), then you could just repeat that ritual meaninglessly until that person was completely stripped of magic permanently. Or you could use other rituals which require similar permanent sacrifices until you achieve the same effect. Keeping a permanently magicless wizard imprisoned is a trivial task, and obviates the need for dementors.
Side Note: That's actually my pet theory on why Dementors as prison guards are acceptable to the public. It could be that governments used to use rituals to permanently strip prisoners of magic before imprisoning them. This would make them a revenue center instead of a funds sink. This would naturally encourage the magical government to find more and more excuses to imprison people, similar to how the 'tough on crime' cycle is accelerated by the for-profit prison systems in some places. A police state would be soon to follow. Then, after a cultural revolution, Dementors were adopted as the less evil option to house criminals. It also helps explain why so many rituals are banned. It's unlikely to be true in HPMoR, but it'd be a nice thought for another fanfic.
similar to how the 'tough on crime' cycle is accelerated by the for-profit prison system in America.
The next discussion thread is here.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 85. The previous thread has long passed 500 comments. Comment in the 15th thread until you read chapter 85.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: