Latest one that I've seen is this guy: http://news.ycombinator.com/user?id=dailyllama , but I see lots of cases all the time, such as this one: http://news.ycombinator.com/item?id=3755037 (who wasn't very happy about being hellbanned).
None of these people are trolls -- one of their posts must have been downvoted early on, and they've tripped some sort of automatic mechanism. Perhaps you shouldn't hellban people like this unless one (or more?) of their posts has been explicitly flagged (rather than just downvoted)?