You would be able to control the part of the network you cut off, but if anyone smuggles in as much as a microSD card with the outside blockchain state and sync

You would be able to control the part of the network you cut off, but if anyone smuggles in as much as a microSD card with the outside blockchain state and syncs it to their client, every client in the segment would see this longer chain (since the outside has more hash power than you) and recognize it as legitimate, undoing all your fraudulent transactions (along with a lot of legitimate ones that never reached the outside network)