Of course we could have stopped 9/11 if we could read the future and could arrest people based on what our foresight predicted they would do in the future.
No, perhaps we could have prevented 911 by allowing the FBI to investigate these people and their activities given that they were known to be part of a terror group that had been killing Americans (and others) around the globe FOR YEARS.
Government is not infallible. I do not demand that government protect me from all disasters. But we knew Al Queda was responsible for many terror acts. That they were here certainly signified something! Perhaps, had they been deported (I doubt they could have been arrested) they would have been spooked enough to stay away. Perhaps they would have come back. But today we could say that at least our government wasn't asleep at the wheel.
But .. if the FBI had taken the information and broken up the cell and arrested Atta - I don't believe 9/11 would have happened - not the way it did. It was still a year away - and the final planning would have gone undone.
Of course, I suppose it's possible that another member of the group would have taken over - but it could have meant fewer attacks.