Where I live the police will not touch it. I know from first hand experience. They will tell you it is a civil matter and that you have to take it to court to have the party removed.
My tenant moved in a crazy girlfriend and then could not get her to leave. He called us wanting us to get her out. Since he had never told us that she was living there, and filled out proper paperwork, she was not on the lease. It was HIS problem. The only choice we had in the matter was to evict HIM for breech of contract which would include all unknown occupants.
She had established residency, he allowed her to move in, so he was the one that entered into the agreement with HER not us. Legally we could not touch her (crazy I know), we could only go after him. However, he could have filed eviction on her himself. How that would have played out in court - I don't know - but this is what our attorney told us was the only option if he wanted to stay.
He too thought it was our responsibility, and blamed US for a decision he made, and wanted us to clean up his mess.
In the end he ended up with a restraining order filed against him by the girlfriend and he was not allowed on the property. She however was still there and had moved in her new felon boyfriend within a few days. We served notice to him and followed through with eviction on him and all other Unknown occupants.
These situations vary by state and local laws/procedures. The best advice, already given above, is to consult an attorney.
If he is on the lease, then it is HER problem, not yours. I am assuming the individuals are over 18? If not that may be a whole other ballgame.