You can lock your class(es) to ensure additional students aren't able to join even if they have the join code. You can quickly unlock/re-lock your class(es) to allow new students to join. Tip: Locking your class(es) will also prevent you from making any accidental changes.
From your "Classes" tab, select the class you'd like to lock by clicking on the bubble to the left of the class name
Click on the "Lock" icon at the bottom of the page:
Need to unlock? Simply click on the red lock and click on the "Unlock" link on the window that appears.