The employee has to get the manager, who is the property's custodian. That person has to formally tell you to leave. A cop can do the formal telling on the custodians behalf. A cop cannot issue a trespass warning on his own. He can't kick you out, and then arrest you for coming back. The property owner, or someone in the position to act on behalf of the property owner would have to initiate the warning. Now, if the owner of this business complied with the mandate, he would back the arrest.