The coordinator of the evaluation (the education administration of your faculty) sets up the start date and end date of an evaluation. When the end date expires, the evaluation is closed automatically. The administration also has the option to stop the evaluation early. The teacher does not have this option.
However, it is possible as a teacher to start and stop a live evaluation. To stop, click CLOSE EVALUATION. For more information, read the article How do I do an in-class evaluation?. If an evaluation is closed, the (QR)code will no longer be visible for students. It does not mean, however, that the evaluation is closed completely and that students no longer have access to it.