Hmmm...I've heard of court orders and injunctions doing this, but now a cop can? I'd be interested in exactly which law permits LEO's - rather than courts - to do this.
Easily. The LEO is enforcing the rights of the property owner to decide who gets to stay on his property.
The process goes like this: The property owner, manager or other representative of the owner asks the police to 'bar' a subject. The police write (or issue) a Trespass Warning, which informs the subject that he/she/it has been ordered to leave the property by someone in control(owner/manager/representative) of that property. The warning states clearly that the owner (manager/representative) does not want that person to return to the property, and if that person does return to the property, she/she/it will be arrested for Criminal Trespass.
Voila, you have been barred.
LawDog