How to Hide Easy Critical CSS Admin Menus

function plt_hide_easy_critical_css_menus() {
	//Hide the "Settings → Easy Critical CSS" menu.
	remove_submenu_page('options-general.php', 'easy-critical-css-settings');
	//Hide the "Settings → Contact Us" menu.
	remove_submenu_page('options-general.php', 'easy-critical-css-settings-contact');
	//Hide the "Settings → Get API Key" menu.
	remove_submenu_page('options-general.php', 'easy-critical-css-settings-pricing');
}

add_action('admin_menu', 'plt_hide_easy_critical_css_menus', 1000000000);

Where do I put this code?