How do I remove my quiz from showing up in Google?

If you marked your quiz "private" it would not show in google. Private quizzes are only visible to those whom you share the URL of the quiz with.

If the quiz was first public and then marked as private, its possible that Google indexed it earlier. We add the noindex and nofollow directive for Google (and other search engines) on private quizzes so next time it refreshes, it should remove the quiz from search (a process that can take a few days or be instantly - just depends on Google). If you need it removed immediatly you can :

1. Password protect the quiz - So even if someone finds it, they can not access it without knowing the password. Keep the password simple and common across all quiz takers. 
2. Copy the quiz, mark the copy private and delete the old one. However in this case you would lose quiz taker's data of attempts, reports & stats.