A picture that has been revoked can be permanently removed from the CMS Exports list.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: Only the built-in administrator can permanently delete 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. 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.