In a single domain, there is NOTHING you can do to prevent someone from accessing a specific share on a server once they have Domain Admin privileges. All you can do is slow them down or prevent them from accidentally accessing the files in question.
You're only effective solution is to set up a second domain and make sure your second admin is not part of the domain admin group of the box/boxes in question. That will stop him dead.
Oh, as several others pointed out in this thread, don't rely on share premissions. Set the actual permissions on the files and directories in question. That way, no matter how I get to them I am forced to deal with the permissions on the files themselves.
Dave