[...]if such a feature was available and the thread was publicly visible, they would just have to log out or go incognito to view it.
It will be easily solved using
extra criteria (Daily Posts, Likes, messages, etc) in specific threads.
ie, -You open a thread and you specify that users need to have at least 10 likes to see that thread.
-some user give you problems, then you decide
ban that user and you selelect:
the user will think,
ok, I'll log out and take a look... but there are criteria applied
then the user will see this nice message:
You must be a logged-in, registered member of this site to perform this action.
Would be awesome have this feature in the future in Xenforo.