General antonyms:
* Conceal: To hide or keep secret.
* Misinform: To give false or misleading information.
* Withhold: To refuse to give information.
* Deceive: To mislead or trick someone.
* Obscure: To make unclear or difficult to understand.
Specific antonyms depending on context:
* If you are talking about informing someone about a specific event: Ignore: To pay no attention to.
* If you are talking about informing someone about a decision: Suppress: To prevent from being known.
* If you are talking about informing someone about a danger: Endanger: To put someone at risk.
It's important to consider the specific situation when choosing an antonym for "inform."