Als in the sense of “than” is followed by that case according to the grammatical context of the sentence. It may be semantically relevant:
Sie liebt das Kind mehr als ihr Mann. (nominative) — “She loves the child more than her husband does.”
Sie liebt das Kind mehr als ihren Mann. (accusative) — “She loves the child more than she loves her husband.”
Als in the sense of “as, like” is claimed by some traditional grammars to require the nominative case: Er verkleidet sich als spanischer Stierkämpfer. (“He dresses himself up as a Spanish bullfighter.”) it may indeed be the most common usage in reflexive constructions, such as in the example given (although als spanischen Stierkämpfer is perfectly acceptable). The mechanical use of the nominative, however, is often ungrammatical by any standards of common usage: Sie kannte ihn schon als jungen Mann (“She knew him already as a young man”; the nominative als *junger Mann would be odd and indeed would suggest the meaning that she knew him when she was a young man). Thus, the same general rule applies as given under (1).