nullify

verb

1. (transitive)To make legally invalid.

   “Near-synonyms: cancel, void, quash”

   “The contract has been nullified.”

   “[T]hey were baffled by tears in moustached sixth-formers, by walls of impassivity in the Lower School, by silent conspiracies which nullified the rules.”

   “Lesbians for Justice therefore appeals to the organizers of the Michigan Womyn's Music Festival to nullify its "womyn-born-womyn only" policy, and to allow the attendance of post-operative male-to-female transsexuals at all future festivals.”

2. To prevent from happening.
3. To make of no use or value; to cancel out.

   “The persuasion that a thing is impossible, at once nullifies endeavour, and like the Turkish "it is fate," torporises activity and exertion.”

   “Blowproof water has given the Navy the weapon it needed to fight fires with helicopters. Previously, the downwash from the rotor blades blew the foam, or whatever agent was being used, off the fire, nullifying its effect.”

   “It nullifies the night / from overkill”