The passwords are of course securely hashed. If the passwords are complex enough, then this should be enough to keep your passwords unknown.
I assume you want to keep someone from clearing the password and then booting up. This is not simple, but you could have a root startup script so when the computer boots, it checks to see if the shadow file is incorrect, and if it is, it deletes, crashes, or otherwise disables the machine or makes the private information inside unreadable. This does not stop the clearing of the password, but gives you an opportunity to take action on the next boot.
But remember, if the machine never boots, the hacker can still read all the files, regardless of the shadow file.
Caveat is of course that you need to write some program to detect malicious changes to the password file, while not ruining you when you make authentic changes. Probably you just want to detect if there were changes since the last known copy but only if that occurred while the machine was offline.