Hi,
I had the same problem, and I found out that everything you ever import or paste into a FLA, is being saved in the library, even if you deleted it from the actual scene.
The solution is to open the window/library, look for the unwanted file and delete it from there. Since I am using Flash 5, and I'm new at this, I suggest that you make a copy of your FLA and try what I suggested on the copy. This way you wont hate me forever for messing up your work. It worked for me, hope it will work for you too.
If this is all wrong, and you find another solution, please let me know.