It's amazing how you always know you are on the wrong side of history when it comes to this. Did you ban a book? Yes? Ok you know where you are now.
Like everything else there is wiggle room, or you could have been dealing with a no-good alternatives situation, but this one test is absolute. You can make a moral justification for temporary slavery, say your nation is being invaded by people willing to genocide your population and you really need people to work for the war effort. Slavery is often considered the ultimate evil but there still exists situations where you could argue it needs to happen for a little while.
You just can never find a situation where ideas were dangerous and must be abolished from a person's head. Heresy is always a victimless crime. It does nothing to anyone else that someone commits wrong-think.
I just find this astonishing. Every other sin of the human race there is some crazy horrible circumstances that I can say it has to be this way because as bad as it is not doing it is worse. Except for one.