Before archiving a class type, you must first remove any schedules associated to that class type (see How to delete a class schedule for more on this).
Navigate your way to Class Types >> Classes >> select desired Class Type.
From the Class Detail page, click the gear icon and then 'Archive'.
Click 'Archive Class Type' once more from the next page, to confirm the removal of this class type:
Note. Archiving a class type will remove this completely. You will not be able to retrieve this class type again once it has been archived.