nullify

To make legally invalid.

Near-synonyms: cancel, void

The contract has been nullified.

To prevent from happening.

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.