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