Since I reinstalled µTorrent I noticed that all my torrents are shared between all users – exactly like in this question.
The problem is, unlike in the question above, I don't want this, and I don't know how undo it.
How can I disable that?
You can change the folders you are using in preferences. Just make sure they're your user folders, where by default other users have no access. If they do, then check permissions on that folder, and remove them if they are set.