The administrator can turn on and off the ability to comment on a node. However, in the content type "Book page" it cannot be tuned off. An autenticated user will always have the opportunity to leave a comment even if the administrators does not want this. This bug also happended on a new content type I created. In the content type "Basic page" (that came with the installation) this works as intended. I have tested this in Drupal 7.2 and 7.4 and they behave similar (tested in http://science-network.tv). It would be great if this bug can be fixed.