It's especially unacceptable if the source is Goggle.  I get that people forget to cite their source sometimes, but you can only forget once or twice over a long period of time before it becomes blatant plagiarism.  As for being reminded to put in a citation after the post was made, that's a tricky one.  I don't really have a problem with it, and it might be more constructive and a positive thing overall than just letting the member get banned--but that's assuming the member learns his lesson from that warning.