You can't cancel a keyup event. You can cancel keydown and keypress events though. In the documentation, notice that under "Event Information", "Cancels" is "No" for keyup:
Using keydown allows you to cancel far more keys than keypress, but if you don't want to cancel until after the key has been lifted, keypress is what you want. Fortunately for you, the enter key is one of the cancellable keys for the keypress event.
