Any persons or firm hereinunder assessed shall have the right to demand of and receive from the treasurer or disbursing office of the agency a refund of such assessment so collected, provided such? demand for refund is made in writing within 30 days from the end of the assessment year which shall be determined by the agency. Should a person or firm hereinunder assessed refuse to pay and does not pay the assessment within 30 days of when it is due and payable, then in such event suit may be brought by the duly certified agency in a court of competent jurisdiction to enforce the collection of the said assessment. (1967, c. 890, s. 16; 1971, c. 642, s. 1.)