Have you ever wanted to change text formats for your site's comments?
In my case, I wanted all comments done only in plain text no matter if it is a signed in user (authenticated) or just an anonymous one. Formats for contents still depend on the original settings - this only affects text formats for comments only.
That's all there is to it!
Now whenever a user needs to drop a comment for your blog post, he/she can only do it in plain text only..
Do the same for your other content types such as book type etc as you desire..