Note: Be careful; there is also a system user account (IIRC it's called "debian-maintainer" or something similar). Under no circumstances should you do anything with that account (i.e. don't change the name or password). Otherwise backups and other important stuff will stop working...

If you are wanting to do this for security then probably a better plan would be to disable Adminer. By default MySQL is bound to localhost so with Adminer disabled, the MySQL root account is not accessible unless you are logged in. And if an attacker can log in, then access to MySQL is the least of your worries...!