An intercept is possible if confirmation of a 9/11-style hijacking is done in time and if fighter assets are ready to scramble and within range.  But as the Soviet experience in the infamous KAL 007 shootdown suggests, knocking a "heavy" multi-engined civil airliner out of the sky may not be as easy as one might think.  On paper the supersonic Su-15 "Flagon" (aka "The Boeing Killer") that did the deed should have had no trouble, as it was armed with both cannon and air-to-air missiles.  But 'twas not so.

In a related matter, some aging U.S. fighter pilots might admit to you that an unwritten instruction during the height of the Cold War opened the option of using their own aircraft in kamikaze attacks upon approaching Soviet bombers if their missiles, rockets or guns proved ineffective.  As one wing or group commander is rumored to have said, "Hey, might as well.  If a Russian bomber drops a 50-megaton nuclear weapon on your loved ones, just what do you expect to come home to anyway?"