I got a comment like that on an older fic just a month or so ago. I was very much annoyed, but in the end, decided to ignore it. I have only once responded to a bad comment, and the conversation that ensued was...not exactly unpleasant, but made me very angry, because the commenter apparently decided that I was a new writer when I was, in fact, not. (Actually, just typing all that annoys me all over again and that was years ago.)