If the owner fails to comply with an order to repair, alter or improve, or, at the option of the owner, to vacate and close the building, the public officers may cause such building to be repaired, altered or improved, or to be vacated and closed; the public officers may cause to be posted on the main entrance of any building so closed, a placard with the following words: "This building is unfit for human habitation or occupancy or use; the use or occupation of this building is prohibited and unlawful."