Disable Gravatar?

Any suggestions to remove gravatar?

Below does not work in child functions.php;

and editing /wp-includes/pluggable.php looks a little complicated ...

function __default_local_avatar()
{
// this assumes default_avatar.png is in wp-content/themes/active-theme/images
return get_bloginfo('template_directory') . '/images/default_avatar.png';
}
add_filter( 'pre_option_avatar_default', '__default_local_avatar' );