The phrase "cast loose" means to release or set free. Antonyms for this phrase would include words like constrain, bind, clutch, hold, and cling to. When we constrain something, we put boundaries or restrictions around it, holding it tight to prevent its release. When we bind something, we tie it up or hold it in place, preventing it from moving. Similarly, clutching, holding and clinging to something keeps it from being set loose. All of these antonyms imply some sort of confinement or restriction rather than the freedom that "cast loose" implies.