The antonym for "coopt" is "exclude." When one coopts someone, they are bringing them into a group or organization, while excluding someone means to keep them out. Other antonyms for coopt might include "disenfranchise" or "marginalize," which suggest a more active form of exclusion, or "dismiss" or "ignore," which imply a lack of involvement or recognition altogether. When considering the antonyms of "coopt," it's important to think about the opposite of bringing someone in or taking them under your wing, and instead pushing them away or leaving them out in the cold.