How can I create myself as SUPER ADMIN so othe admins cannot delete my user until I allow them to?

I'm just now beginning to use WPMU. I have built MANY WP site and will continue to do so, hopefully, now installing and using WPMU for various things.

I have many clients whose websites are completed, but payments are not yet finished. I need to maintain my admin access to their sites until their payments are complete (and possibly beyond, for updates and fixes purposes). I do not want other admins to be able to edit or remove my user.

How can I accomplish this?

Thank you!