Is there any case when it's correct to pronounce the word police with the stress on the first syllable:
It is possible to put the main stress on the first syllable of police in some varieties of English. When the first syllable of police is stressed, the vowel is not a schwa. It is the "goat" vowel or "long o" sound: /ˈpolis/ or /ˈpoʊlis/ (both of these phonemic transcriptions are identical).
There is no way to classify this pronunciation as indisputably "correct" or "incorrect" in a global sense, because there is no consensus about how to define "correct pronunciation". That said, Mark Liberman (in the post linked below) suggests that "the initially-stressed [pronunciation of police] seems to have become stigmatized, and have been abandoned by many better-educated or more upwardly-mobile people." There are many specific speakers who would never stress the first syllable of police in any context. So it's acceptable for a non-native speaker to always say /pəˈlis/, with stress only on the second syllable.
The pronunciation /ˈpolis/, with stress on the first syllable, is supposed to occur for some speakers in the Southern US, according to the following sources:
Straight Dope Message Board discussion: Poh-lice, or police?
"The Southern Stress on the First Syllable in Words like Cement and Police", by Grant Barrett, A Way with Words
"Thanksgiving Variation", Mark Liberman, Language Log