The erection or maintenance by any person, firm, or corporation of any sign for which a permit is required by this part without a valid permit or renewal thereof issued by the department 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 erecting or maintaining, without a valid permit or renewal thereof, any sign for which a permit or renewal thereof is required by this part. 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.