When removing a song from a library, iTunes will first ask whether you wish to delete it and then whether you wish to move the selected songs to the Recycle Bin. If you do so, the actual file will be deleted from your computer. Therefore, neither you nor the other users will be able to use it anymore for any library. So, if you intend to remove a song from one library but wish it to be still available to other users or to yourself for your other libraries, make sure to click Keep Files when prompted by iTunes.