[peterwwillis]

  >  Constructive, no. Especially to someone who didn't understand enough not to do this in the first place

That's the whole point of the comment - to tell them not to do this! If we knew they did this on purpose just to piss us off, we wouldn't make the comment to begin with. We'd just put "Asshole."

In general it's not good or useful to present comments in a negative way, because negative comments cause a negative reaction from the intended recipient. But they're still getting the information that there was something specific they need to improve upon. So it's still constructive, it's just also rude.