There is not a way to turn comments on or off for all published posts and pages at once. However, you can use the bulk edit feature to change many posts or pages at a time.
- Go to the Posts -> All Posts screen in your blog dashboard.
- Select the posts you would like to modify or check the box next to “Title” column at the top to select all posts on that page.
- Select Edit from the Bulk Actions drop down menu.
- Click the Apply button.
- Change the options for Comments and Pings to Allow or Do not allow.
- Click the Update button.
- Repeat with your Pages.
For more info, check out this site: http://en.support.wordpress.com/enable-disable-comments/
I found a YouTube video that explains how to do one page or post at a time here: