The Spark demo site lives at http://demo.sparkdrupal.com/
With anyone able to login and have a play with the new functionality it can get very messy, cluttered, confusing and broken very quickly. I have a couple of suggestions to help with this:
a) Each visitor is shown their own copy of the demo site.
b) The current single copy of the site is retained but it will automatically reset(/reinstall) itself to a default state at the end of every day. I would suggest a countdown timer in the corner to inform users how long remains until the reset.
Option (a) could be implemented in a multitude of different ways, however as time is probably better spent elsewhere at present I would suggest opting for option (b) for now.