In order to allow your objects to be garbage-collected, you can either remove all listeners from it or use weak event listeners instead (don’t forget to remove other references to it as well). To make a listener weak, you must set the 5th parameter of the addEventListener method to true, like this:

var btn:Button = new Button();
btn.addEventListener(MouseEvent.CLICK, clickHandler, false, 0, true);

But you have to be very careful where you use weak event listeners. When you instantiate an object in a function’s scope and then add weak event listeners, your object could be garbage-collected before it had a chance to dispatch these events. Example:

private function getStuff():void {
    var service:HTTPService = new HTTPService();
    service.url = "get-stuff.php";
    service.addEventListener(ResultEvent.RESULT, resultHandler, false, 0, true);
}

In this case, you should either avoid using weak listeners and remove them manually, or add a reference to your object outside of the function’s scope to prevent it from being garbage-collected too early.