When we talk about antonyms for the phrase "acts as middle," we can use words like "separates," "divides," and "isolates." While acting as a middle person, one is essentially bridging the gap between two parties, connecting them to form a coherent whole. However, in some situations, such mediation may not always be desirable or necessary. Some situations call for a more aggressive approach where there is a need for clear and distinct boundaries. In such cases, "separates" may be a more appropriate antonym for the phrase "acts as middle" since it denotes creating conscious divisions between two entities. Finally, "isolates" may be used to describe situations where parties are intentionally kept apart rather than brought together.