function smartwp_remove_wp_block_library_css(){
wp_dequeue_style( 'wp-block-library' );
wp_dequeue_style( 'wp-block-library-theme' );
}
add_action( 'wp_enqueue_scripts', 'smartwp_remove_wp_block_library_css' );
Gutenberg
Disable Gutenberg editor
Disable the Block Editor to return to the Classic Editor.
add_filter('gutenberg_can_edit_post', '__return_false', 5);
add_filter('use_block_editor_for_post', '__return_false', 5);