We don't waste our time with calling the police around here, we take care of the problem ourselves, and no one is the wiser. With my excavator, I can dig a deep hole in the matter of minutes.
I would call the police. No cops where I live are going to allow squatters to stay where they don't belong. Plus, I live in a gated community and my neighbors would have booted them out already.
In my state squatters, if they (the squatters) cannot prove they have a right under adverse possession laws, can be criminally charged with trespassing and be immediately removed by law enforcement. Not all states have such logical laws. KNOW YOURS.