A picture that has been revoked can be permanently removed from the CMS Exports list. To do so, first make sure the picture is revoked, then click on the orange Trashcan icon next to it. To avoid mishaps, you cannot remove a picture without revoking it first.
Note: Permanent deletion of exports is only available to users who have been granted the administrative permission Manage Exports through group membership.
Generally, only items that have been exported by mistake and not yet published should be permanently deleted, since removing the picture from the exports altogether will create broken links in the CMS. Simply revoking an asset, on the other hand, will generate a "pretty" "Media not available" icon and should be the preferred method of making assets unavailable.