View in stack doesn't act as expectation

//Main page
class MyDelegate extends WatchUi.BehaviorDelegate {
	function onBack(){
        if(mController.isRunning()) {
            mController.stop();
            WatchUi.pushView(new Rez.Menus.BeforeExitMenu(), new BeforeExitMenuDelegate(), WatchUi.SLIDE_UP);
        }
        WatchUi.requestUpdate();
        return true;
    }
}

//Before exit menu
class BeforeExitMenuDelegate extends Ui.MenuInputDelegate {
	// When the before exit menu is shown, I wanna to disable the back button.
    function onBack(){
        return true;
    }
}

When the before exit menu is shown, I wanna to disable the back button. But the menu is stuck, how to cope with this issue?

I have a main page. A confirm menu will be shown when back button is pressed.Now I wanna disable the back button. But the menu is stuck, how to cope with this issue?