Hide comments of a specific post and show a predefined text instead. Only the post author and commenter (registered user) himself can see his comment. And the post author can enable/disable this mask and show original comments, anytime!
In case of you want to hide comments of your blog posts, or show them to registered members only; probably this is the only plugin that can help you achieve this!
General Settings Page
Default Settings Page
Post Submit Box with Mask Comment Option
Mask Option Customization