Because I made it possible for the user to launch a window from the main interface, logic said that I had to let them close it. I had to embed into the movie clip a button which triggers a shutdown of the panel. For the 2advanced windows I designed a single square in the right-hand corner to serve as the close trigger. Still in the .swf file of the externally loading movie, I had to insert the button within the timeline of the movie clip. I added an action to this button under its Instance Properties:

On (Press)
  Begin Tell Target (“/level1”)
    Unload Movie (4)
  End Tell Target
End