Negate (verb)

1. To deny the existence or truth of something.
2. To cancel out or make ineffective.

Origin:

From latin negāre ("to deny, refuse").

Examples:

1. The new evidence negates his previous testimony.
2. His argument is negated by the facts.
3. The treaty was intended to negate the previous agreement.
4. The new laws were designed to negate the effects of the previous administration's policies.
5. His decision to quit negates his chances of winning the election.