The antonyms for the phrase "let someone out" could include "trap," "confine," "imprison," "detain," and "restrict." These words imply the prevention of someone's freedom or the deliberate attempt to keep them inside. Additionally, other antonyms could include "hold on," "keep in," "restrain," "confine," "shut in," and "imprison." These words suggest a lack of agency or control over one's movements and decisions, as well as an overall restriction within an enclosed space. Ultimately, the antonyms of "let someone out" describe situations where individuals are forcibly held or prevented from leaving, rather than given the freedom to go where they please.