I don't think you want to go there- nearly all transatlantic flights from or to the USA use Canadian airspace, and the Gander Oceanic control centre manages about half of all transatlantic flights (half the north atlantic). A pissing match would harm Canada, harm the US economy even more, and would not prevent a single attack.
I know .. I just wanted to point out that it can be done and Canada doesn't have us by the *alls.
If we are attacked again and if it came from Canada because their lack of security on the war on terror .. I'm guessing many in this country wouldn't be to upset if we cut off Canada