How do I grant irrevocable privacy from user with sudo access?
Basically all files stored locally are accessible by an admin user (or any user who can run a root shell). This also includes locally stored/cached files held in Dropbox, Google Drive, iCloud etc. 

If a user wants to prevent an admin looking at his/her file, he/her should create an encrypted DMG using Disk Utility, should not store its password in the Keychain and should take extra care to always unmount the DMG whenever he/she walks away from the computer.