How can I prevent one Admin from deleting another Admin??
Any attempt to restrict what an admin can do is an exercise in futility. Some very limited restrictions are possible but they are nothing more than speedbumps for anyone beyond the novice level. Giving such individuals admin access on a business computer would be asking for trouble.
By design all admin accounts have essentially unrestricted control over the computer. Whatever access they do not have they can grant themselves. Any change made by one admin can be changed by another. All admin accounts are equal. The built in Administrator account is a little different but not relevant to your issue. Any admin can delete, change the password, and make any other changes to any account without knowing the password to that account. All of this is by design.
All users with admin accounts must be trusted to use their powers for good. You have to work out your issues by means of policies known to all admins. They cannot be enforced by any settings on the computer.