A Spanish Spanish speaker (one from Spain) would never, ever say "nunca jamas." Such a double negative would be unnecessarily reiterative. They'd - depending on mood, context, color coordination - choose between either "nunca" or "jamas".

I agree that (Spain) spanish grammar is very precise, but that is why I added a side note saying "hasn't anyone ever heard in spain:
"subir arriba", or "bajar abajo"... it is a butchery of spanish grammar, BUT it is ok with me =)
In my opinion, the main difference between spanish from spain and spanish from latin-american is the ability that spaniards have to bend grammatical rules in order to bring a personalized expression by using the language as a metaphor . That is why I must say that even though "nunca jamas" is a double negative, it should be ok depending on what one is trying to express.
"Nunca Jamas" might be rarely said in Spain, but at least there is always something like "Entrar adentro"
=)
gabriel