The antonyms for the phrase "coming on board" are "leaving," "departing," "quitting," or "resigning." These words indicate a separation or removal from a particular situation, which is the opposite of "coming on board" and symbolizes joining a group or organization. Additionally, antonyms like "rejecting," "refusing," or "declining" connote a refusal to accept an offer or invitation to become a part of something. These antonyms indicate that the person does not want to get involved or be a part of the group or organization. Overall, antonyms for "coming on board" are words that represent a refusal to join or leave a particular situation.