непреклонный

1) General subject: Catonian, adamant (adamant to entreaties - непреклонный к мольбам), cast iron, dour, grim, hardshell, inexorable, inflexible, intransigeant, intransigent, lockstep, never say die, never-say-die, quite firm, relentless, rigid, rock ribbed, rock-ribbed, steadfast, steely, stern, sticky, stiff, strong-minded, unbendable, unbending, uncompromising, unmoved, unpersuadable, unrelenting, unstoppable (о стремлении), unyielding, determined, diehard, die-hard, stubborn, staunch

2) Poetical language: stark

3) Bookish: obdurate

4) Diplomatic term: indomitable, uncomplying

5) Makarov: immovable