How to stop key_down event in flex?

I'm trying to do this: handle key_down event, and stop the propagation, but no joy.
I handle the key_down event globally through the stage:

this.stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDown, true, int.MAX_VALUE);

and in keyDown function:

###########
do my work
###########
if (needToStop)
    event.stopImmediatePropagation();

Actually, I'm developing a foreign language (vietnamese) input method for my flex application, because flash textfield had had famous bug about unicode/foreign character input, although it can display unicode character beautifully.Start Free Trial