I think it might help to delete your Limewire preferences folder. This must be done with Limewire FULLY closed. You will loose all your settings including your shared options, (and possibly incomplete files) which in your case should not be a bad thing. When you restart Limewire you will need to reconfigure it. I advise using preferences, rather than the Library to add or remove shared folders.
The preferences folder is at HD>Users>Your Name>Library>Preferences>Limewire simply drag to the trash
Let us know if this helps.
The sharing of downloaded files from your last session should stop next time you start limewire, but they should not be shared atall if you have checked not to share them. If you have checked to share them, you cannot afaik stop sharing them without restarting Limewire. (or of course deleteing them
)