Unload content of a MovieClip

programming flash
1

Hi!

I have 4 MC working as a button and when I click one of them I want to unload the content of a MC that I use to load external’s MovieClips and load a new one inside this “stage” MovieClip. I am using a function that recieves which external swf to load.
My question is how to unload the previous content of the empty movie clip?

Here is my function:

function loadSWF(swf:String) : void
{
   var loader : Loader = new Loader();
   var url : URLRequest = new URLRequest(swf)
   
   loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS,onProgress);
   loader.contentLoaderInfo.addEventListener(Event.COMPLETE,onComplete);
   loader.load(url);
   mainBox.holder.addChild(loader);
}