The phrase "turn loose" refers to the act of releasing or freeing someone or something from control or restraint. The antonyms for this phrase would be "constrain," "restrain," "fetter," "shackle," "handcuff," and "imprison." These words describe actions that involve confining or controlling someone or something, often against their will or natural course. When we turn someone loose, we are giving them the freedom to act and make choices on their own. But, when we impose constraints, we limit or even eliminate those freedoms. Therefore, the antonyms for "turn loose" reflect the opposite of freedom and self-determination.