inflexible
adj.unable to change or bend easily. You use this to describe someone who sticks to their rules or plans no matter what happens.
adj.lacking the ability to change or bend easily; rigid in attitude or behavior. Often used to describe systems, people, or policies that resist adjustment.
“The company has an inflexible policy on late arrivals.”
“The old building's inflexible structure made it impossible to add a new wing without tearing down the original walls.”
“While the initial strategy was sound, the team's inflexible adherence to the original plan prevented them from adapting to the sudden market shift.”
Borrowed from Middle French inflexible, from Latin īnflexibilis. See also in- + flexible.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.