If you're referring to the act of turning something around a central point:
* Fix: To make something stationary.
* Secure: To fasten something firmly in place.
* Immovable: Unable to be moved.
* Stationary: Not moving.
If you're referring to a swivel chair or a swiveling device:
* Fixed: Not able to turn or rotate.
* Static: Not moving or changing.
* Non-rotating: Not designed to turn or rotate.
If you're referring to the act of changing direction quickly:
* Straighten: To move in a direct line.
* Direct: To move in a specific direction without turning.
* Constant: To maintain a consistent course without changing direction.
The most appropriate antonym will depend on the specific situation and the intended meaning.