When you are viewing a classroom, at the bottom of the page there is a button to Delete Classroom. Clicking it will display a prompt asking if you are sure you would like to delete the classroom. 

Deleting a classroom cannot be undone, so only delete it if you are sure you would like it permanently removed. 

If you would like to keep the classroom in tact, but prevent any classroom members from using it, you can click on Edit Classroom Settings and mark the classroom as Inactive instead.