General Opposites:
* Retain: To keep possession of something.
* Hold: To keep something in a particular place or state.
* Keep: To continue to have something.
* Restrict: To limit or control something.
* Confine: To keep something within limits.
* Bind: To tie or fasten something.
* Capture: To catch and hold something.
* Suppress: To stop something from being expressed or known.
Opposites Related to Physical Objects:
* Secure: To fasten something firmly.
* Attach: To join something to something else.
* Lock: To close something securely.
* Seize: To take hold of something forcefully.
Opposites Related to Information or Ideas:
* Conceal: To hide something.
* Withhold: To keep something back.
* Suppress: To stop something from being expressed or known.
* Censor: To remove parts of something that are considered offensive.
* Obscure: To make something difficult to understand or see.
Opposites Related to People:
* Imprison: To put someone in prison.
* Arrest: To take someone into custody.
* Detain: To keep someone from leaving a place.
* Control: To have power over someone.
* Subjugate: To bring someone under control.
Opposites Related to Legal or Medical Situations:
* Retain: To keep someone in custody.
* Restrict: To limit someone's freedom of movement.
* Confine: To keep someone in a particular place.
* Prohibit: To forbid something.
* Withhold: To not give something that is due.
Remember that the best antonym will depend on the specific context in which you are using the word "release."