After you've opened your blog book find and select Posts under the Contents section. This will bring you to a page that lists all the posts and comments in your blog book. To remove all comments for one post just mouse over the that post and select Remove All to remove all comments from the post.
To remove individual comments, select the post and then just scroll to the comments section located at the bottom of the page (you may need to click the up arrow to make it visible), find the comment you'd like to delete, and click remove. If you decided you want to delete all of the post's comments from here you can simply scroll to the bottom of the comment section and click Clear Selected Comments.