General antonyms:
* Conceal
* Hide
* Suppress
* Deny
* Silence
* Withhold
Antonyms related to specific meanings of "proclaim":
* Disavow (to deny responsibility for or connection with)
* Recant (to withdraw a statement or belief)
* Disprove (to show something to be false)
* Debunk (to expose the falseness of something)
* Downplay (to make something seem less important)
* Ignore (to pay no attention to)
The best antonym will depend on the specific situation and the intended meaning of the word "proclaim."