adamant
adamantine
brassbound
diehard
fossilised
fossilized
hard-core
hard-line
hardline
inexorable
intransigent
ironclad
muscle-bound
ossified
rigid
rock-ribbed
semirigid
stiff
unadaptable
-- unbending
at a standstill
cast-iron
certain
changeless
closed to
constant
determinate
determined
dogged
dour
dyed
established
fated
fateful
firm
fixed
flinty
frozen
fundamentalist
grim
hard
hard and fast
headstrong
hidebound
immobile
immotile
immotive
immovable
immutable
impersuadable
impersuasible
impervious
implacable
impliable
inalterable
indefeasible
indomitable
inductile
inelastic
ineluctable
inescapable
inevasible
inflexibility
inflexibleness
inflexibly
haughty
imperturbable
intractable
resolute
steadfast
stern
stubborn
unalterable
unflinching
unforgiving
unyielding
flexible