Run PostgreSQL without admin / superuser
This answer is about PostgreSQL.

You cannot drop a superuser unless you are a superuser yourself.
Nobody can drop the bootstrap superuser (normally postgres) because he owns the system objects.

Resetting the password won't prevent a user from logging in.

Keep the superuser around and don't allow it to connect.

For that, you could add the following lines at the beginning of pg_hba.conf:

host  all   postgres  0.0.0.0/0  reject
host  all   postgres       ../0  reject
# if you are truly paranoid and want to forbid local connections
local all   postgres             reject


Don't forget to reload PostgreSQL after that.