Whenever the Building Commissioner orders a structure or premises, or part thereof, to be vacated, he shall cause to be posted at each entrance to such structure or premises, or part thereof, a placard warning of the existence of such vacation order. No person shall deface or remove such placard without the written permission of the Building Commissioner until the repair or demolition is completed. No person shall enter or use any structure or premises so placarded except for the purpose of making the required repairs or demolishing the structure or premises.