Section 15. Prior to its expiration date, a certificate of registration may be suspended or revoked by the director, in accordance with the procedures and on the grounds set forth in section eleven, or may be terminated by voluntary surrender by the registrant. Further grounds for suspension or revocation are: (a) a violation by a registrant or any agent or employee of the registrant of any of the provisions of this chapter and (b) the payment of any amount from the fund because of the conduct of a registered contractor or subcontractor where said contractor or subcontractor has not repaid the fund in full, including the appropriate amount of annual interest.