"Eles não podem mostrar o seu rosto."
In general, native speakers of English would say, "They cannot show their faces." This should be accepted as a valid translation for the sentence above.
I agree with hkysonjr.
Then there is the possibility, if I'm not mistaken, that "o seu" could mean "his", "hers", "yours", "its" although those would require a more specific context. Is this not so? Grateful if a native PT speaker would comment.