An employer shall not hire for work in a plant currently involved in a strike, lock-out, or labor dispute an employee who, to the employer’s knowledge, is unaware of the existence of the strike, lock-out, or labor dispute. An employment agency shall not refer persons to employment in a plant currently involved in a strike, lock-out, or labor dispute without adequate notice to such persons that there is a strike, lock-out, or labor dispute in the plant at which the employment is offered.