Uhm.. why does an admin
have to have access to that safe? I mean, an admin knowing his or her tools of the trade does not have to worry about "illegal" blocks or the likes and if a player wants to store away stuff out of anyone's sight, so be it. And in case it is obvious, that action has to be taken due to someone hacking/duping/whatever.. well.. it CAN be destroyed with TNT and it's not really a shame to use this, as the person owning the a-safe has done something very wrong in the first place and probably is not a healthy addition to your servers' community. Though if I were said admin, I'd probably spawn in an ICBM and place a sign down the hole saying "That'll teach ya!".
I mean, if it's just a case of "I r teh admin. I must haz teh powah.".. thats a bit dodgy for an admin in the first place. ^^