How do I disable default Tkinter key commands?

I'd like to implement my own key command. However when I do, it does both what I tell it and the default command. How do I disable the default command, so that my command is the only one that runs?

This is on Windows 7, BTW.

Put return 'break' at the end of your event handling function. This tells Tkinter not to propagate the event to default handlers.