1- You are on the wrong track. I really dont understand why you guys need necessitity to grant someone Site Admin priviliges if you have doubts that he can delete your account?

Simply make them Super Moderators and you'll be perfectly fine.
2- However I'm pretty sure you wont listen to me and still ask for the hack so here you go lol:
edit admin/user.php:
find:
if ($HTTP_POST_VARS['action']=="kill") {
after that add:
if ($userid==1) { show_nopermission(); exit;}
Now no one can delete the account of user 1.
But I still INSIST you really consider not to make anyone Site Admin. Preventing them not deleting your account is pretty meaningless when he is able to:
- change your password
- delete all your messages
- change your user group and make you a banned user
- delete all your templates in your board
- delete all other accounts in your board
- make all users site admins
- thousands of other bad things he can do in your admin cp
And to preventing them from doing all these is only and easily achivable if you remove their Site Admin priviliges