Completely disable single user mode

A very simply way to disable single-user mode is to edit /etc/rc.boot (as root or with sudo) and in the section that prints "file system mounted read-only", simply add a line that says reboot Now, anytime the computer is booted in single-user mode, it simply reboots itself. 
This is totally non-bypassable because Apple disables any usable keys in the begining of the rc files.