Antonyms for assent

agreement:

dissent, disapproval, disagreement, repudiation, dissension, veto, opposition, denial, refusal, rejection.

assent:

disagree.

agree:

oppose, resist, refuse, decline, argue, reject, dispute, differ, protest, repudiate, disagree, deny, disapprove, disallow, object.

noun:

dispute, decline, rebuff, oppose, reject, contradict, refuse, gainsay, contend, scorn, demur.