obdurate

unable to be persuaded or moved emotionally; stubborn; unyielding.

No number of pleas and bribes would get him to change his obdurate attitude.

Geithner plainly has no patience for what he describes as the obdurate unwillingness of colleagues to subordinate their desire for superficial impact to the larger vision.

In their valuation of the distribution of grace, theologians distinguish somewhat sharply between ordinary sinners (among whom they include habitual and relapsing sinners) and those sinners whose intellect is blinded, and whose heart is hardened, the so-called obdurate sinners (obcaecati et indurati, impaenitentes).

Note that you can build your GRE vocabulary via Vocab Assistant app hosted in facebook messenger.