inexorable
Meaning: incapable of being persuaded or placated
Usage: Although I begged for hours, Mom was inexorable and refused to let me stay out all night after the prom.
implacable
Meaning: incapable of being appeased or mitigated
Usage: Watch out: once you shun Grandma’s cooking, she is totally implacable.
irrevocable
Meaning: incapable of being taken back
Usage: The Bill of Rights is an irrevocable part of American law.
indomitable
Meaning: not capable of being conquered
Usage: To be honest, Jim, my indomitable nature means I could never take orders from anyone, and especially not from a jerk like you.