Actionscript 3 :: Removing Object From The Display List?

I have an application that uses a main class to control other MovieClips, adding and removing them as needed, most of them are separate screens or sub menus.

When I leave the main menu and come back to it later from another screen, certain animations and roll over buttons still play, which I don't want, I basically need the main menu screen to reset every time it is seen.

I read up on this and found out that removing the child doesn't necessarily remove it from the memory.

I tried setting the mainmenu to null before moving onto a different screen but this threw up an error, stating that the parameter child must be none null.

how to completely kill my mainmenu when it is not needed.

public function confSubMenuOneScreen():void {
subMenuOneScreen = new SubMenuOne();
mainmenu = null;

[Code].....

This is the example of code that removes the menu and adds another screen, with the mainmenu = null code that throws up the error.