annul in WordNet English dictionary

1. cancel officially
   "He revoked the ban on smoking"; "lift an embargo"; "vacate a death sentence"
   revoke  annul  lift  countermand  reverse  repeal  overturn  rescind  vacate 
   
2. declare invalid
   "The contract was annulled"; "void a plea"
   invalidate  annul  quash  void  avoid  nullify