adding text/links to comments closed line

When you close comments on a post or just have them off to visitors I would like to have it not just say comments closed but something like - Sorry you can not leave comments as a visitor. Please consider subscribing to get access to full stories and lots of great features on our site.

Is there a plug in that does this or do I have to edit a file to make it happen?

I am using WP/BP and a child theme.

I appreciate your help. Thanks.