adamant
Meaning: impervious, immovable, unyielding
Usage: Though public pressure was intense, the President remained adamant about his proposal.
obdurate
Meaning: unyielding to persuasion or moral influences
Usage: The obdurate old man refused to take pity on the kittens.
recalcitrant
Meaning: defiant, unapologetic
Usage: Even when scolded, the recalcitrant young girl simply stomped her foot and refused to finish her lima beans.