A picture that has been revoked can be permanently removed from the CMS Exports list. First, ensure the picture is revoked, then select the trashcan icon next to it. To avoid mishaps, you cannot remove a picture without revoking it first.

Note: Permanent export deletion is only available to users with the administrative permission to Manage Exports.

Generally, you should only delete items that have been exported by mistake and not yet published, since removing the picture from the exports altogether will create broken links in your CMS. On the other hand, simply revoking an asset will generate a Media not available icon, which should be the preferred method of making assets unavailable.