Disable single user mode using lilo

In your LILO configuration section, write something like:

image = /vmlinuz
        label = Linux
        root = /dev/hda1
        password = wingedlizard
        restricted
        read-only

Then chmod this file 600 (so nobody but root can read it) and re-run /sbin/lilo.

The “restricted” keyword means that LILO will stop and ask for a password if you try to boot this kernel with _any_ keywords such as “1″ or “single” or “init=/bin/bash”. A password won’t be required during normal (no-added-keywords) boots.