Unloading external swf

Hi,
I’ve managed to load in an external swf into an mc called “box”, using the following code(AS3):

var pictureLoader:Loader = new Loader();
var pictureURL:String = “xxxxx.swf”;
var pictureRequest:URLRequest = new URLRequest(pictureURL);
pictureLoader.load(pictureRequest);
box.addChild(pictureLoader);

Now I can’t get rid of it!

How do I code a button to unload it, and stop all the sounds in it from continuing?

Hope someone has a moment to set me straight …

Thanks for looking.