Formal:
* Invalidated: This emphasizes the loss of legal or official validity.
* Annulled: This suggests a formal cancellation or voiding of a contract or agreement.
* Abrogated: This implies the complete ending or revoking of something.
* Repealed: This is used specifically for laws or regulations.
* Voided: This is a general term for making something ineffective.
Informal:
* Canceled: This is a more casual way to say nullified.
* Ruined: This suggests that something has been completely destroyed or made useless.
* Undone: This implies that something has been reversed or made ineffective.
* Made void: This is a simple and straightforward way to say nullified.
The best synonym will depend on the specific situation and the intended meaning.