Merriam-Webster Collegiate Dictionary - nullify

 transitive verb  (-fied; -fying)  Etymology: Late Latin nullificare, from Latin nullus  Date: 1595  1. to make null; especially to make legally null and void  2. to make of no value or consequence Synonyms:  ~, negate, annul, abrogate, invalidate mean to deprive of effective or continued existence. ~ implies counteracting completely the force, effectiveness, or value of something a penalty nullified the touchdown. negate implies the destruction or canceling out of each of two things by the other the arguments negate each other. annul suggests making ineffective or nonexistent often by legal or official action the treaty annuls all previous agreements. abrogate is like annul but more definitely implies a legal or official act a law to abrogate trading privileges. invalidate implies making something powerless or unacceptable by declaration of its logical or moral or legal unsoundness the court invalidated the statute.