The antonyms for the phrase "let hang out" are "restrain," "control," and "suppress." When we let someone hang out, we give them the freedom to do as they please and behave without any restrictions. By contrast, when we restrain someone, we limit their freedom by holding them back or controlling their behavior. Similarly, when we suppress someone, we prevent them from expressing themselves or behaving in a certain way. All three antonyms suggest a degree of control or restriction, which is the opposite of the freedom and independence implicit in the phrase "let hang out.