The Delete Element... menu item allows you to delete one
  or more elements from the library.

  You cannot delete an element whose generations have any
  existing reservations.  Additionally, the element cannot
  be a member of a group, nor can one of its generations
  belong to a class or be under review.

  If any generation of an element is reserved, you must
  unreserve or replace it before you can delete the element.
  If the element belongs to any groups or has generations
  in any classes, you must first remove them by doing the
  following:

  1. Pull down the Maintenance menu.
  2. Choose the Remove submenu.
  3. Choose the Element... or Generation... menu item.

  Then provide the necessary information in the subsequent
  Remove Element or Remove Generation dialog box.

  If the element is set with the reference copy attribute,
  CMS deletes the corresponding reference copy file from
  the reference copy directory.