In CA, the owner would probably be arrested for doing that. The freeloaders that do this stuff are experts at gaming the system. It could drag out for years.
True grifters know and work the system, but they rely on the other party using the system. Forcing them out and then simply not acknowledging that they were ever there changes the game. They are then forced to prove that they were squatting. The squatter is then in the same position as if they had just called the police and demanded entry into a home at random.
Treat them as an insane nut job. If they break in, then treat them as you would an insane nut job breaking in.