"Unstar as interesting"
Solution 1:
If I read "unstar as uninteresting," I would understand something was before starred as uninteresting; in the same way, I would understand "undelete as spam" as the opposite operation of "delete as spam."
Solution 2:
I understand what you're getting at. You are asking whether it should be (unstar) (as uninteresting) or (un(star as interesting)). Since the adverb as XXX modifies the verb here (and the verb being unstar), it would be unstar as uninteresting.
This, of course, is assuming that the verb to unstar itself is acceptable.