Section 11. A final judgment or decree rendered in any civil or criminal proceeding brought by the attorney general under this chapter to the effect that a defendant has violated this chapter shall be prima facie evidence against such defendant in any action or proceeding brought by any other party against such defendant under this chapter as to all matters respecting which said judgment or decree would be an estoppel as between the parties thereto; provided, however, that this section shall not apply to consent judgments or decrees, or settlements which have not been fully litigated to their conclusion, none of which shall constitute either an admission or prima facie evidence. If a final judgment or decree is rendered in any civil or criminal proceeding brought by the attorney general under this chapter to the effect that a defendant has violated section four, the commonwealth or any city or town may void, rescind or cancel any contract to which the violation pertains or relates, and which was entered into with said defendant.