The City Officer shall serve by registered or certified mail, return receipt requested, postage prepaid, a copy of said abatement order forthwith upon those persons who have filed written statements of objections. The abatement order shall specify that unless the nuisance is removed and abated within seven (7) days from the date of the mailing, said officer will abate the same and will, if necessary, enter upon or into the private property of said person without further notice or liability therefor in order to abate said nuisance.