Go to Settings on the sidebar. At the top is an option to archive a course. This will shut down the class and hide it from your main screen.
Learn how to find your archived classes here.
If you want the class fully deleted you will need to contact Peergrade.
Archiving a Live Session
To archive a Live Session, go to the "Settings" tab on the Live Session interface. Under the sidebar menu you will find a button to archive / unarchive and delete a live session.