You could hide some contents in posts, then those who have not replied to any posts will not see them.
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Disclose-Secret
Only shows WordPress articles to other users if the user meets certain access criteria.
SMS paid content
This Plugin allows you to hide any content on your blog, that will be visible only after user sends sms message.
Revisions
Provide Versioning, Preview and Rollback ability on Pages and Posts.
Content Pay by Zong
Content Pay by Zong allows you to monetize your blog content using Premium text messaging.
jqs-random
A simple random phrase plugin that can be used anywhere, via PHP or shortcodes.
OronjoWordPressPlugin
The Oronjo Wordpress Plug allows you to easily import/update your Oronjo content into Wordpress.
Oronjo, sell your content on your own site.
TFS LOLcat
A text filter that will convert your posts and comments to LOLcat-style language.
Text Filter Suite
Advanced filtering functions for WordPress, including the Talk Like a
Pirate Day filters.