> The Ancient Egyptians has a solution to this issue: the admins were
> buried with the pharaoh.

I am more blunt with clients when this comes up.

If you are this untrusting of your admins fire them right now. Three

1) If you cannot trust them this far, you cannot trust them at all.
Fire them now.

2) No matter what you put in place to block them, if they are good
enough they will get around it. So if you do not trust them this far,
fire them.

3) If they cannot get around them, they are not very good at their job
so fire them.

I like Bob's thought.