Yes, that is where I get stuck. Of there was a deterministically harmful book, like the play ,"the king in yellow", which drove every reader violently mad, then I would want it banned. There are unquestionably books and ideas that are statistically harmful to the society most of us want to live in. I just don't trust anyone to be the arbiter of what gets included in that category. But I live in a low trust society, so maybe it is a solvable problem?