Sparkbooth can be locked to prevent guests from opening settings and minimizing the application. To enable this feature, go to Settings > Keyboard > Lock Application. A password can also be set to in order to toggle the lock to provide an extra level of security.
For those using a touch screen without a keyboard, there is a touch point in the lower rigth that can be used to toggle the lock.
If you forgot your lock password, you will need to reset your settings back to default.