Comment Expirator let's you close comments, pingbacks and trackbacks on your posts, pages and custom post types on an individual basis.
Comment Expirator allows to schedule closing of comments, trackbacks and pingbacks on an individual basis at a date or time of your choice. It can be activated on any post type that allows comments. This is good for preventing spam on old posts. The plugin lets you chose if trackbacks and pingbacks should also be closed at the same time.