I think in cases where the original comment appears to be made in good faith, it's OK to explain what's wrong with it/what to do instead. That's how people learn.
(Obvious spam/flames/etc. should just be downvoted, ofcourse.)
It seems like about half of ilovefood's comments are of the "wow! great article!" type. I'm not sure whether this indicates that comments saying "don't do that" are likely to be helpful or not...
(If it were 100%, I'd conclude that it's a would-be spammer's account, building up a bit of history before starting to try to fill HN with spam. But I don't think that's what's going on here.)