countermand
     n : a contrary command cancelling or reversing a previous
         command
     v : annul by recalling or rescinding; "He revoked the ban on
         smoking"; "lift an embargo"; "vacate a death sentence"
         [syn: {revoke}, {annul}, {lift}, {reverse}, {repeal}, {overturn},
          {rescind}, {vacate}]
Source: WordNet® 2.0