Follow these steps to unshare a file or folder.
If you have shared the file with a member of the University it's possible to unshare the file to particular people, rather than needing to globally unshare the entire file.
If the file has been shared outside of the University, it is not possible to selectively unshare it from people. Instead, you must unshare it from everyone.
As a workaround for selectively unsharing a file to people outside the University, consider changing the password protection, and then only informing those you would like to have access about the change.