Abort OS X download initiated through App Store?
You could just go to the Applications folder on your Mac, and find the App that is downloading, and delete it from there.  Once you do this, it should pop up a box asking you if you want to cancel the download.  Just select yes and cancel it.

I've done this before and it worked fine.  Usually, I just let it fully download, then uninstall/remove the App with an App called Clean My Mac, or App Zapper.  Both are great apps for uninstalling/removing Apps and other stuff from you Mac completely/fully.

Hope this helps :)

-James