Can't be done.
Most you can do is make it readable only by the user that owns it. If something is readable, it's also copyable.
There are ways to make HTML code readable but not copyable via a web browser...but as for a file on some type of *nix box, no. Same thing, I guess with .doc(s) as well, but once again, that's within the application it's meant for, not on the OS level.