If I'm not mistaken, there are anti-air guns mounted on the White House roof with people at them 24/7. However, I really doubt they could take out a 767, and even if they could, like dawheat said, it'd have enough momentum to keep going. PLUS, even if they somehow managed to stop it, that wouldn't be good, because then it'd fall in the middle of the city and kill a ton of innocent people.