The government, the army, the police are still necessary, as are certain rules of conduct in society.
Stealing things from people who live in decentralized world will be made difficult.
There will be stuff like alarms, Cameras will be watching, there will be very tough defenses, and quick means of escape (known to only the citizens) . Things will be reordered frequently to make detecting this security secrets very difficult.
Besides, having too much physical properties will be discouraged amongst the citizens. The properties should be easy to move around and easy to decoupled..
Much of our wealth will be in decentralized world, so that they can't be easily stolen. Decentralized Insurances will also help incase of theft or accidents