iTunes U User Guide for web
Generate a new enrollment code in iTunes U for web
If you receive too many requests for enrollment in your private course or your course enrollment code was released without your permission, you can prevent students with the current code from requesting enrollment by generating a new code for your course.
Important: Generating a new course enrollment code invalidates the current code. Currently enrolled students and pending requests are unaffected. However, students who have not yet requested enrollment must use the new code.
Generate a new course enrollment code
In iTunes U
for web, click the Students cell for your course in the dashboard.Click Admin > Admin, then click Enrollment.
Click “generate a new code”, then click Generate New Code in the pop-up pane.
iTunes U generates and displays the new enrollment code for your course.
Note: From the dashboard, click 
in the row for the course you want, choose Enroll Code, click 
in the pop-up pane, click “generate a new code”, then click Get New Code.