The phrase "hammer out a deal" means to come to a decision or agreement through negotiation and discussion. Antonyms for this phrase could include "breakdown" or "fail to reach an agreement," "dismiss" or "reject a proposal," or "resist" or "refuse to cooperate." Alternately, one could describe a situation in which an agreement was reached quickly and easily as "smooth" or "seamless," as opposed to the implication of hard work and effort in "hammering out" a deal. The context and tone surrounding the phrase will inform the most appropriate antonym for each situation.