【Inflexible】 ,  【inexorable】 ,  【obdurate】 ,  【adamant】 ,  【adamantine】  mean not to be moved from or changed in a predetermined course or purpose.

All are applicable to persons, decisions, laws, and principles; otherwise, they vary in their applications.

【Inflexible】  usually implies firmly established principles rigidly adhered to; sometimes it connotes resolute steadfastness, sometimes slavish conformity, sometimes mere pigheadedness <society’s attitude toward drink and dishonesty was still  【inflexible】  —Wharton >  <a morality that is rigid and  【inflexible】  and dead —Ellis >  <arbitrary and  【inflexible】  rulings of bureaucracy —Shils >

【Inexorable】 , when applied to persons, stresses deafness to entreaty <more fierce and more  【inexorable】  far than empty tigers or the roaring sea —Shak. >  <our guide was  【inexorable】 , saying he never spared the life of a rattlesnake, and killed him —Mark Van Doren >  When applied to decisions, rules, laws, and their enforcement, it often connotes relentlessness, ruthlessness, and finality beyond question <nature inexorably ordains that the human race shall perish of famine if it stops working —Shaw >  It is also often applied to what exists or happens of necessity or cannot be avoided or evaded < 【inexorable】  limitations of human nature>  < 【inexorable】  destiny>  <you and I must see the cold  【inexorable】  necessity of saying to these inhuman, unrestrained seekers of world conquest … "You shall go no further" —Roosevelt >

【Obdurate】  is applicable chiefly to persons and almost invariably implies hardness of heart or insensitiveness to such external influences as divine grace or to appeals for mercy, forgiveness, or assistance <if when you make your prayers, God should be so  【obdurate】  as yourselves, how would it fare with your departed souls? —Shak. >  <the  【obdurate】  philistine materialism of bourgeois society —Connolly >

【Adamant】  and  【adamantine】  usually imply extraordinary strength of will or impenetrability to temptation or entreaty <Cromwell’s  【adamantine】  courage was shown on many a field of battle —Goldwin Smith >  <when Eve upon the first of men the apple pressed with specious cant, O, what a thousand pities then that Adam was not Adam-ant —Thomas Moore >