It's more ethically dubious, but in theory he could do it to one person who could cast the Patronus Charm, have them testify under Veritaserum that they were no longer capable of it as a consequence of what Harry told them, then Obliviate them of the specifics- even if it doesn't restore their ability, at least it prevents them from affecting anyone else. Then the issue is proving they're not Occlumens.
But it's not like I'm advocating this idea, or anything.
ETA: Oh! He could do it to Amelia Bones!
ETA: Oh! He could do it to Amelia Bones!
Or even Dumbledore.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
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: one, two, three, four, five, six, seven, eight, nine, ten.
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: