nullify in WordNet English dictionary

1. declare invalid
   "The contract was annulled"; "void a plea"
   invalidate  annul  quash  void  avoid  nullify 
   
2. show to be invalid
   invalidate  nullify 
   
3. make ineffective by counterbalancing the effect of
   "Her optimism neutralizes his gloom"; "This action will negate the effect of my efforts"
   neutralize  neutralise  nullify  negate