I would suggest simply editing it into a block quote and adding the source, as long as it's still helpful.
Flagging doesn't seem to be the right course of action, since flagging should generally mean "I need a moderator to do something I can't." At this point, that's basically only deletion, and if it's helpful, then deletion isn't appropriate.
If there is brief, pertinent information to be included from the external link, then quoted content in the form of excerpts is fine, as long as the original reference is included. It may even be a good thing to be included in an excerpt*, since we don't want answers going stale. Of course, if there is non-trivial information to be gleaned from a large portion of the article, the author should summarize in their own words, rather than copying, as wholesale copies do not qualify as fair-use, as Dori mentions in the comments below.
Answerers should put anything that is not in their own words into block quotes, like I've done on this question where I, too, used information from an About.com link.
* This is more important on sites like Skeptics, where paraphrasing can cause more trouble