Aside from Open Firmware/EFI passwords, you can configure your Mac so that the root password must be entered in order to access Single User Mode. If your root account is disabled, then it is impossible to enter the root password, and Single User Mode cannot be started.

To do this, the console and ttys must be marked as insecure in /etc/ttys:

1. Log in as administrator
2. Open Terminal
3. cd /etc
4. sudo cp ttys ttys.old (backs up previous ttys config).
5. sudo pico ttys
6. Replace all occurrences of the word "secure" with "insecure" at any lines that do not begin with a "#"
7. Exit, saving changes.

These instructions are from the Apple Mac OS X Security Configuration manual.

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