1) Click the 'Class Open' Switch so it changes to the 'No' Position. This setting will only affect student access, not staff access which will remain the same. 

2) This has the effect of preventing student access to the class. The student will still be able to see the class appear in their class list but it will appear greyed out and if clicked on it will take the students to a page informing them that the class is inaccessible right now.