You can find this in your Class Settings on the main dashboard
Once you have clicked class settings , a pop up will appear and you can check the box Close Registration
The second way to close registration is in your class dashboard press (+) Add Students
This will bring a pop up to the screen where you can choose how to add your students
If you have chosen to have students create their own accounts a pop up box will appear and allow you to check the close registration box.
Once you've checked the box you will get a notification saying that your registration is closed .