It shall be unlawful for any person to obstruct, impede or interfere with any officer, agent or employee of the city, or with any person who owns or holds any estate or interest in any building or structure, or any portion thereof, which has been ordered to be repaired, vacated and repaired or demolished, or with any person to whom such building or structure has been lawfully sold pursuant to the provisions of this article, whenever any such officer, agent, employee, purchaser or person having an interest or estate in such building or structure is engaged in repairing, vacating and repairing or demolishing any such building or structure pursuant to the provisions of this article, or in performing any necessary act preliminary to or incidental to such work, as authorized or directed pursuant hereto. (Ord. No. 1401 (NCS), § 2.)