Depending on how you've set up your work spaces and how you asked your students to create accounts, deleting a classroom might equate to deleting student profiles and erasing student access. This is a possibility since some students sign up only with a classroom access code. Part of our site's design is to make it so that students don't need an email address to participate, which makes account management a bit complicated. This is a long way of asking you to think twice before deleting a classroom altogether.
The best practice, before deleting a classroom, is to ensure that all students have had a chance to download and back up their work and that both you, and your students, have availed yourself of everything that you wanted or needed from the experience. This includes winner badges, downloadable goodies, or anything that ties to their profile and requires being logged in.
Once you're ready to delete, open the classroom you'd like to delete from the Dashboard, the My Classrooms dropdown, or the View All Classrooms page. Then, go to that classroom's Admin section. Click the wrench in the upper right corner to delete that class.