How to Hide the "Simple Author Box" Admin Menu

function plt_hide_simple_author_box_menus() {
	//Hide the "Appearance → Simple Author Box" menu.
	remove_submenu_page('themes.php', 'simple-author-box');
}

add_action('admin_menu', 'plt_hide_simple_author_box_menus', 11);

Where do I put this code?