I think Mpamaegbu likely intentionally plagiarized his post. The post in question is several years old, and I am not sure if he was participating in a signature campaign at the time. IMO he is sufficiently established of a forum member that a temp ban, rather than a permaban would probably be in order.

Although none of the cases in the OP apply, there are potential situations in which plagiarism could be "unintentional". For example, someone could copy content from multiple sources to make a well-rounded argument, and not properly cite said sources. If someone can quote many sources to make a coherent post that is informative and on-topic, I would be okay with allowing them to receive a warning. The sources would need to be diverse enough to show the person was putting in substantial effort into his post, and the post needs to be something that the average user reading the subject thread would find interesting, and not believe to be nonsense.