- adamant, adamantine, inexorable, intransigent - not capable of being swayed or diverted from a course; unsusceptible to persuasion; "he is adamant in his refusal to change his mind"; "Cynthia was inexorable; she would have none of him"- W.Churchill; "an intransigent conservative opposed to every liberal tendancy"
Antonym: flexible (indirect, via inflexible)