function plt_hide_gosmtp_menus() {
//Hide "GoSMTP".
remove_menu_page('gosmtp');
//Hide "GoSMTP → Settings".
remove_submenu_page('gosmtp', 'gosmtp');
//Hide "GoSMTP → Test Mail".
remove_submenu_page('gosmtp', 'gosmtp#test-mail');
//Hide "GoSMTP → Support".
remove_submenu_page('gosmtp', 'gosmtp#support');
}
add_action('admin_menu', 'plt_hide_gosmtp_menus', 11);