Can't answer for Netware, but can for WinNT/2K.
The short answer is you can't (for and administrator)(unless you encrypt everything).
If you change the attributes of a directory to hidden, one must only change the folder options to show hiddenfiles.
If you change permissions to specifically deny access to a file or folder, an administrator still has ways to get at it. Administrators have a user right to take ownership of any object. Once ownership is taken over, permissions can be changed to allow access. Denying access to a folder will prevent anyone denied access from seeing the contents of the folder, but not the folder itself.
You could even try denying the administrator the ability to log-on to the local machine using Group Policy (Win2K) or System Policy (NT4), but all an administrator needs to do is set the same policy at the domain level and it will overright and take precedence over any local policy.
Even if you encrypt everything, all that does is prevent anyone else from opening a file or folder. It doesn't prevent listing the file or folder(i.e. seeing they exist).
If your goal is to prevent access, encrypt it. If your goal is to prevent ANYONE from seeing that the file/folder exists, stick it on a floppy and carry it with you.