“here’s what I suggest: Place a placard at the entrance”
They violated the verbal commands, why not a placard?
Not sure about DC, but in many states a “posted” placard is required to support a trespass charge. From what I understand, it is more difficult to prosecute a person for trespass who lawfully entered the premises, but refuses to leave, than it is to prosecute someone who unlawfully entered the premises in the first instant.