YES! I just tried to do this and was sad to see this is not available.

But a thread can always be moved to another category & group - like the spam posts are moved tio deleted / hidffen ones..

And ( I am not sure - this is a question more to our CSS Gurus) with CSS it should be possible to "hide" the switch to show the "hidden" groups too.

But the search command will show all…anyway..

Best usage is to use instead a forum the comment-less forum with pages only ( sub - pages ( like posts to threads).. as used on our admin forum:
http://community-playground.wikidot.com/admin-forum