Load your items to the same clip. When you load a new movie onto a clip that already has a movie on it, the previous movie gets automatically unloaded to make way for the new movie.
Either that or try [AS]targetClipInstanceName.unloadMovie()[/AS]
You are going about the method sorta wrong. It is best and more efficient to create each of your sections in an external .swf file and use loadMovie() to call them in.
This prevents the user having to load anything they won’t view which saves on load time, and it keeps your content seperated and easy to edit, and you would only need one clip in your main movie to load to and you won’t have this overlay issue