dour
firm
fundamentalist
hard
hidebound
impliable
inexorable
inflexible
iron
ironbound
ironclad
ironhanded
muscle-bound
obdurate
obstinate
orthodox
purist
puristic
puritan
puritanic
relentless
rigid
rigorist
rigoristic
rigorous
rockbound
stiff
straightlaced
straitlaced
stubborn
unbending
uncompromising
unrelenting
unyielding