hard-line
hardline
adamant
adamantine
cast-iron
dour
firm
flinty
fundamentalist
grim
hard
hard-core
hidebound
immovable
immutable
implacable
impliable
inelastic
inexorable
inflexible
intransigent
iron
ironbound
ironclad
ironhanded
irreconcilable
muscle-bound
obdurate
obstinate
orthodox
procrustean
purist
puristic
puritan
puritanic
relentless
rigid
rigorist
rigoristic
rigorous
rock-ribbed
rockbound
single-minded
steely
stern
stiff
straightlaced
straitlaced
strict
aggressive
defiant
fanatical
manly
merciless
outspoken
passionate
resolute
ruthless
steadfast
stubborn
unflinching
unyielding