How to disable root user account in configuration.nix?

When I install NixOS from scratch, users.users.root.hashedPassword = "!" doesn’t do anything. The installer still prompts for root password, and I can still log in as root w/ said password.

Any way to prevent the installer from prompting for password?

$ nixos-version
21.05.1790.382039c05a1 (Okapi)