I think you can locked grades in the grade book ie. override all grades with the current grade and then open the quiz.
There is also https://moodle.org/mod/forum/discuss.php?d=328970 which may be helpful if you want to develop a plugin.
Re: Moodle quizzes: is it possible to disable grading after a certain period, but let the quiz open?
by Daniel Thies -
Number of replies: 0