How do you setup a secure demo site?

General Information

In /includes/version.php look for:

/** @var string Whether site is a production = 1 or demo site = 0 */
var $SITE = 1;
/** @var string Whether site has restricted functionality mostly used for demo sites: 0 is default */
var $RESTRICT = 0;

For a demo site it is advised to following:

/** @var string Whether site is a production = 1 or demo site = 0 */
var $SITE = 0;
/** @var string Whether site has restricted functionality mostly used for demo sites: 0 is default */
var $RESTRICT = 1;

$SITE = 0
// Allows multiple user logins with only one account. By default Joomla! 
// allows only one active session per account as a security feature.

$RESTRICT = 1
// Disables those logging in, both Front-end and Back-end from changing 
// user details - like password and username

These settings are used on the official demo site http://demo.joomla.org

You should also make all files and folders nonwriteable - especially the configuration.php file. Also recommend you setup an automatic cron job that refreshes the database at a set interval (in our case 60mins) from a db script.