A website I worked on last year, recently had a wave of media spam comments hit my inbox. There is no easy way within WordPress to turn off commenting for media items, especially if you have >100 items and just want it DONE! The solution I found was a quick SQL query:
UPDATE `wp_posts` SET `comment_status` = 'closed' WHERE `post_type` = 'attachment' AND `comment_status` = 'open';
Super simple (if you have command line mysql or PHPMyAdmin). Hope this helps!