mouseEnabled = false;
mouseChildren = false;
Example:
var dispObj1:Sprite= new Sprite();
dispObj1.graphics.beginFill( 0x000000, 1 );
dispObj1.graphics.drawCircle( 50, 50, 25 );
dispObj1.graphics.endFill();
dispObj1.buttonMode = true;
dispObj1.addEventListener(MouseEvent.CLICK, clickHandler);
var dispObj2:Sprite= new Sprite();
dispObj2.graphics.beginFill( 0xff0000, .5);
dispObj2.graphics.drawRect( 0, 0, 100, 100 );
dispObj2.graphics.endFill();
//disable MouseEvent for dispObj2
dispObj2.mouseEnabled = false;
dispObj2.mouseChildren = false;
addChild(dispObj1);
//dispObj2 over dispObj1
addChild(dispObj2);
function clickHandler(event:MouseEvent):void
{
trace('Hello');
}