There are a fair number of people online who seem to have forgotten that they are writing to another person. They say whatever they want, without regard for feelings. More than likely, they wouldn't do that if you were standing face-to-face with them, but they not only lack manners, they lack imagination.
When this happens to me, I don't address what they say at all, but the tone that they have used in saying it. It does help to remind them that they should be thinking as if they were real grownups.