In the sense of providing information:
* Conceal: To hide or keep secret.
* Misinform: To give incorrect information.
* Deceive: To mislead someone.
* Withhold: To refuse to give information.
* Suppress: To prevent information from being known.
In the sense of making someone aware:
* Ignore: To pay no attention to.
* Disregard: To treat something as unimportant.
* Overlook: To fail to notice.
* Neglect: To fail to give attention to.
In the sense of being informed:
* Ignorant: Lacking knowledge.
* Uninformed: Not having knowledge of something.
* Unaware: Not being conscious of something.
The best opposite for "inform" will depend on the specific situation and the intended meaning.