As3 script issue

What you can do is add the event listener with the key release and remove the key listener until the drop finishes...

var speed:Number = 10; /* falling speed */

stage.addEventListener(KeyboardEvent.KEY_UP, keyReleased);

function keyReleased(e:KeyboardEvent):void {
       if (e.keyCode == 84){
            stage.removeEventListener(KeyboardEvent.KEY_UP, keyReleased);
            ball_mc.x = 250;
            ball_mc.y = 0; /* position starts */
            ball_mc.addEventListener(Event.ENTER_FRAME, moveDown);
      }
}

function moveDown(e:Event):void {
      e.target.y += speed;
      if(e.target.y >= 350){
            ball_mc.removeEventListener(Event.ENTER_FRAME, moveDown);
            stage.addEventListener(KeyboardEvent.KEY_UP, keyReleased);
      }
}