Since the main feature seems to be that a template can't be modified or changed, here are some synonyms for *unchangeableunchangeable
predetermined and not able to be changed
which suggested another one
established or decided in advance
speaking of which...
preset (or pre-set according to MS Word)
set or adjust (something) in advance of its use
and last but hopefully not the least
arranged beforehand, in advance
Note: I've adjusted dictionary definitions of preset and predetermined as verbs, because the dictionary stated that in participle form these can be used as adjectives