The new addition to the Actionscript 3.0 API is unloadAndStop:
This new ActionScript 3.0 API extends the Loader.unload API to help unloaded content use zero CPU and potentially unload from memory. After calling unloadAndStop on loaded content the player will attempt to stop all playback of MovieClips (recursively), and remove references to as many objects as possible (Sounds, NetStreams, EventListeners, etc.) that were not cleaned up by the unloaded movie. This will make it far more likely that the Garbage Collector will collect the memory used by this loaded file. This is useful when unloading unknown 3rd party content.