Formal:
* Invalidate: This is a common synonym that emphasizes making something legally or officially void.
* Abrogate: This implies formally ending or revoking something, often a law or agreement.
* Annul: Similar to "invalidate," this suggests making something legally void.
* Void: This is a very general term that implies something is legally or practically ineffective.
* Cancel: This suggests ending something or making it no longer valid.
Informal:
* Wipe out: This implies completely eliminating something.
* Undo: This emphasizes reversing the effects of something.
* Counteract: This implies negating the effect of something.
* Neutralize: This suggests making something ineffective or harmless.
Other:
* Render ineffective: A more descriptive way to say "nullify".
* Make void: Another straightforward way to express the idea.
* Destroy: This is a strong synonym that implies complete destruction.
* Debunk: This implies proving something false or untrue.
The best synonym for "nullify" will depend on the specific context and the intended meaning.