Just because it's an open canvas doesn't mean you have to be a jerk about it. It's kinda like smashing up someone's Lego contraption when they leave the room because it's stupid and pointless Lego anyways and you can do it anonymously so why not.
A temporary IP ban voted on by the other currently online users would help solve things, although it could be heavily abused, and the ability for (registered) users to 'protect' their own work woulld be good too, except people would make a huge meaningless block beside something else and protect it, so.
Eh, it's not really solvable I guess. Arseholes who want to ruin everyone's fun will always find a way to do it.