Say 32% of the node don't agree with several transaction. Separate that 32% of the nodes into its own economy/system.
This is obviously a response to my post What happens when network is split for prolonged time and reconnected?.
I agree this issue will have to be addressed in-case of massive network disconnection. Some sort of marker for bitcoins and transactions made in the separate network (that have been broadcast and verified with respect to its separate network), perhaps?
If they're malicious, they're isolated from the real bitcoin network. If they want to build their own bitcoin economy, that's fine too.