Except that forensics will allow us to identify the fraudulent block chain, and ban all the relate computers. "Shut it down" really just means "force the Bitcoin community to step it up".

You can't easily "ban" these things, since it's a decentralised P2P system. The whole point is to prevent anyone (original creators, banks, a government, etc.) from being able to interfer with it. It does mean the system fails like that.

Everyone is free to ignore whatever block chains they want to. I can refuse to honor blocks built off what I consider fraudulent block chains.

Would you able to trade with people who hadn't ignored those bloc chains?

How do you ban all the related computers?

The government can already shut it down. They can make laws. There are lots of laws about money laundering and record keeping, and paying taxes that can be used against BitCoin.