You can try to stop it in the pre_comment_on_post hook
add_action('pre_comment_on_post', 'no_wp_comments'); function no_wp_comments() { wp_die('No comments'); } I use this when I use Facebook comments instead of Wordpress comments.
Here is a similar example with an anonymous function:
add_action('pre_comment_on_post', create_function( '','wp_die("No comments");')); but I prefer the first example, it's easier to modify.