7 nullify

nullify
v
(pt, pp -fied) [Tn]
1 make (an agreement, etc) lose its legal force 使(协议等)失去法律约束力.
2 make (sth) ineffective; counteract 使(某事物)无效; 抵销
    How can we nullify the enemy´s propaganda? 怎样才能抵销敌人的宣传?