New rooms that were previously can be deleted if you no longer need them. Note that default rooms cannot be deleted so the option will be greyed out. 

To start, click on Rooms:

Find the room that you want to delete and click the delete icon to the far right of the room name.

A confirmation message will show (the roster will also be deleted - reports and quizzes will not be deleted) - click Yes to confirm. Keep in mind that this is a permanent action and cannot be undone. 

If there's an activity running the room, you won't be able to delete it. You'll see the message below:

To finish an activity, go to the Results tab in your account and click the Finish button. Check out this guide to help you on how to end an activity.