If you have shared a game code with your students, it is possible that an unknown student puts in an incorrect code and stumbled into your quiz. We understand that this is not an ideal situation, hence we added support for "classes" on Quizizz. If you assign quizzes with classes, then only your students can join your games. When you assign with classes, there are no game codes, the assigned games directly show up in your students' Quizizz accounts. Learn more about classes here.
Also, you can always delete students from reports to get rid of uninvited participants.