The ability to delete items from the library is something we've been talking about, so it's good to hear you've run into this, too.
Currently, the way to get old items out of your library panel is to switch to code view, expand the project in your project navigator panel, expand the src folder, images and swfs will be in the assets folder, components in their own folder. Select the desired item and use the context menu to delete it.
Be careful with this, though, it's possible to break your project if you delete the wrong assets. It's kind of a 'secret back-door', so I wouldn't recommend relying on this technique...
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