This plugin is meant to disable comments by default on any new post type, so it’s more of a preemptive strike against comments, while still giving you the flexibility to turn them on each post. It does not affect previously created posts.
I’ve actually got it in the works to have all previous posts’ comments turned off if the user wants it, but haven’t yet had the time to fully develop it.
I haven’t tried this plugin before, but it may accomplish what you are trying to do: http://wordpress.org/extend/plugins/disable-comments/
I know this is an old question, but I just wanted to let you know that I added the feature to disable all comments easily today in the newest version.
Just go to the settings page, and there will be some buttons at the bottom that do what you’re requesting.
