There was a huge chain of events that lead to 911, no one person or one thing could have stopped it.

I suppose if you want to be technical, armed Air Marshalls could have stopped the hijackings from reaching the point they did.

The lists of names would definitely have helped, so would have the capture of Osama Bin Laden in 1998, so would have the fortifying of the Cockpit doors, and I guess just as equally if we were all Muslim and/or stopped supporting Israel it may have not happened.

There's a thousand ways you can look back and say how it could have been prevented, but really, when it comes down to it, no one thing can guarentee that it wont happen again.

Well, except for the will of the American people. I don't see American Civilians on any flight allowing Terrorists to take control of planes ever again, at least not for any prolonged period of time.