[molmalo]

I think there is a simple solution for this:

Just remove all the links to the file contained in public (shared) folders. If the file is in a private folder, maybe it's a private backup, so you should not delete it. You may even display an alert to the user when he logs in, asking to confirm if they hold rights over that file (yes/no). If they click on No, you delete that link too. If they click yes, well... you are not a cop, so you are not obliged to investigate if the user is telling the truth. Anyway, being in a private folder, the copyright owner will never know the file is there. And you can still mark that file (or hash) for never being able to be set as public again in the future.