Guns have been outlawed for years. Did that stop the criminals? Now knives are to be banned. Will that stop the criminals? The only people harmed by these rules are the law abiding folks who use these tools responsibly. Criminals by their very nature do not obey laws!
Pilots with side arms in their bags would have stopped at least one of these hijackings, (in my opinion!). The only reason these terrorists got away with this, is they knew noone else had weapons!