Ninjas are the most deadly weapon in the world. A single ninja can destroy an entire police department without breaking a sweat. Just imagine how many police departments it would take to defeat an evil ninja spirit if it decided to possess the body of Lucinda Dickey.
Thanks to Ninja III: The Domination, we now know the answer - as many police officers as you could get to distract the evil spirit until Shô Kosugi showed up.