Moodle calculates a percentage of the points total, but it seems to do so ignoring the quizzes that were never attempted. These however should count with a 0. So my question: can I change the calculation of the percentage so that quizzes that have closed count as 0 if never attempted? Alternatively, can I simply remove the percentage from the students view?
You can change what students view by going to Grades>Course Grade Settings. You'll find settings for the User Report half-way down the page; toggle "show percentage" to HIDE. If you've left the original defaults, it's set to SHOW.