Blogger is adding excellent features. You can now set a "comment policy" for blogs that is shown to any visitor who intends to write a comment on any of your blog posts.

You can also add external hyperlinks and bold text to highlight the important points of your comment policy.
To add your comment policy, open the Blogger dashboard and goto Settings -> Comments. Type your HTML text in the box that says "Comment Form Message". Save Settings.

When a visitor opens your comment page, he will see this policy just above the comment textarea.

The comment policy of this site says: "Comments posted on Digital Inspiration are moderated and will be approved only if they are on-topic and not abusive. Avoid including website URLs in your comments"