annul
     v 1: declare invalid; "The contract was annulled"; "void a plea"
          [syn: {invalidate}, {quash}, {void}, {avoid}, {nullify}]
          [ant: {validate}]
     2: annul by recalling or rescinding; "He revoked the ban on
        smoking"; "lift an embargo"; "vacate a death sentence"
        [syn: {revoke}, {lift}, {countermand}, {reverse}, {repeal},
         {overturn}, {rescind}, {vacate}]
     [also: {annulling}, {annulled}]
Source: WordNet® 2.0