There are people who are allowed on your property, even with the “no trespassing” signs. And yes, if you tell them to leave, they have to leave. But they can come back. And no, I’m not talking about the police.
And you all know that with a “no trespassing” sign, first you have to tell them to leave. If they return (above exception), THEN they are trespassing and you should call the police or take whatever action you feel is responsible, always with the law in mind.
Who are these people you speak of?