Using Disqus and disabling WordPress comment spam

I have Disqus comments running with very little spam but bots are still able to post spam to the WordPress comments database. I’ve turned off the Disqus sync to WP. The IP address block in WP does not work.

I tried this in functions.php but they still get around it:

// Block WP comments
add_action('pre_comment_on_post', 'block_wp_comments');

function block_wp_comments() {
wp_die( __('Sorry, comments are closed for this item.') );
}

IS there anything I can do to stop them posting?