What they were doing this week was not trying to break into a system and steal anything but more like the demonstrators did in Seattle a couple of months ago. They blocked the street and entrances to the buildings (Yahoo's site) and regular customers and visitor were unable to enter. They do this is a constant stream of messages aimed at the site that serve no purpose other than to keep the servers occupied.

There are countermeasures that can be taken, but usually they depend on blocking certain known users or blocks of users from getting in in the future. A determined group of obstructionists can overcome this.

Just one of the things we have to put up with in a free society. There will always be vandals, graffiti artists, and panhandlers on the streets making life a bit less pleasant.