If you want to stop people from doing this on your computer, you can require that they have the root password to use single-user mode.
To require entry of the root password for single-user mode:
1 Log in as an administrator.
2 Start the Terminal application, located in /Applications/Utilities.
3 At the prompt, enter the command: $ cd /etc
4 To create a backup copy of /etc/ttys, enter the command:
$ sudo mv ttys ttys.old
5 To edit the ttys file as root, enter the command: $ sudo pico ttys
6 Replace all occurrences of the word “secure” with the word “insecure” in the
configuration lines of the file. Any line that does not begin with a “#” is a configuration
line.
7 Exit, saving changes.