Synonyms:
* Control: To exercise authority or power over someone or something.
* Curb: To check or limit (something).
* Hold back: To prevent from moving forward or acting.
* Restrict: To limit or confine something.
* Suppress: To prevent something from being expressed or made known.
Antonyms:
* Encourage: To give support or confidence to someone to do something.
* Empower: To give someone the power to do something.
* Liberate: To set someone free from confinement or restraint.
* Unleash: To release something that has been restrained.
* Permit: To allow something to happen.
The best synonym or antonym for "restrain" will depend on the specific context in which it is used.