In the Updates section of the Mac App Store, if the user chooses to update an app or apps, the interface offers the ability to pause the download of the update, but seemingly not to cancel it. Canceling the download is possible by holding down the option key. The "Pause" buttons change to "Cancel" buttons.
[kirkmc adds: I don't have any current updates to be able to test this. I don't think this stops the update from generating a notification after you cancel the download, but I'm curious as to what happens. Post your experiences in the comments.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20130413220340684