"nullify" in the verb sense
1. invalidate, annul, quash, void, avoid, nullify
declare invalid
"The contract was annulled"
"void a plea"
2. invalidate, nullify
show to be invalid
3. neutralize, neutralise, nullify, negate
make ineffective by counterbalancing the effect of
"Her optimism neutralizes his gloom"
"This action will negate the effect of my efforts"
Source: WordNet® (An amazing lexical database of English)
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