The maintenance by any person, firm, or corporation of any sign which is not authorized by this part and which was not lawfully in existence on October 6, 1971, is declared to be a public nuisance. In addition to the remedies provided for in this part or which may otherwise exist under the laws of Georgia, the department is authorized to bring an equitable proceeding to enjoin any person, firm, or corporation from maintaining any sign which is not in compliance with this Code section. It shall not be necessary for the department to allege and prove that there is no adequate remedy at law in order to obtain the equitable relief provided for in this Code section.