How To Hide The Visual And Html Editor Completely?

For posts:

add_action('init', 'my_custom_init');
function my_custom_init() {
    remove_post_type_support( 'post', 'editor' );
}

See Codex. For custom post types that you register, you can specify what ‘features’ it supports when you register it it use the ‘supports’ arguments.

For custom post types that are not registered by you can use the above with ‘post’ replaced by the custom post type name.