The words "set motion" refer to the act of initiating or starting something. Antonyms for this phrase would therefore involve words that convey the opposite action, such as "halt," "stop," or "cease." These words imply the act of putting a stop to or ending something that has already been started. Other antonyms may include "recede," "retreat," or "withdraw," which suggest a movement away from a starting point. Using antonyms can help to clarify the opposite meaning of a phrase, and can be especially useful when contrasting one action or concept against another.