I have raised two children and this a topic that I have dealt with a lot while they were growing up. If you tell a young person that they don't need to read something, it only peaks their curiosity (even when you explain why) and makes them want to read it even more. Better to explain to them why you don't agree with it, that it isn't real, and let them read it for themselves. Once they have finished the book, openly discuss the pros and cons of it with them.
Early on in my Christian walk I was so determined that my children would not do anything wrong. I became a bit too strict, not allowing them to fully be who they really were. There are things that I would change about the way I raised my own children, but in spite of me, they have both turned out wonderfully.