Simple Security is a simple (As its name suggests) freeware security program. Drop it in your Startup Items folder, and, on startup, the user will be prompted for a password. If the user cancels the dialog box or attempts unsuccessfully 3 times to enter the password, the Macintosh will shut down.
The default password is "Password". To change the password, open up Simple Security in the AppleScript Script Editor, and change what is inside the quotes on line six.
