This is a plugin called Autoclose developed by Ajay D’Souza which manages 3 things in your wordpress posts, which are -
1. Comments
2. Pingbacks
3. Trackbacks

Here is what you can do with Autoclose plugin -

• Closes comments on posts or pages that are a particular number of days old.
• Closes the incoming pingbacks/trackbacks to the blog posts.
• Closes pingbacks/trackbacks on blog posts or pages which are particular number of days old.
• You can choose to leave a few posts allowing the comments/pingbacks/trackbacks.
• Option to delete the post revisions.
• Delete the comments, separate from trackbacks/pingbacks.

How to install the plugin?
1. Download Auto-Close Comments, Pingbacks and Trackbacks.
2. Extract the contents of autoclose.zip to wp-content/plugins/ folder. You should get a folder called autoclose.
3. Activate the Plugin in WP-Admin.
4. Goto Options > Auto-Close to configure.

You can download AutoClose wordpress plugin from Ajaydsouza.com