When the O.P. here innocently inserted a comment asking, "One down-vote. Why?" I thought I was reading the start of a pretty good suggestion.
I expected the rest of the comment to read something like this:
One down-vote. Why? If this person is not a new user, and this user has been exhorted more than once to include the source, then I believe that's enough to justify a downvote on the plagiarized answer.
However, downvotes can be mystifying by themselves, so I cannot recommend this course of action unless an accompanying comment is also provided, one that says something to the effect of:
If you include the link to your source in your answer, I'll gladly remove my downvote
with the link in your comment going to the source that was copied. That would call the user out, yet offer the individual a chance to fix the problem. Done repeatedly, this might lead to a longer-term change of behavior, where the sources get cited upon first draft (which is what I think most folks would ultimately like to see achieved).
By leaving an accompanying comment, others (who are giving this user perhaps undeserved rep points) would get clued into the game, perhaps becoming less likely to upvote someone else's writing. Then again, presumably, the user is doing research to find the answer, and some may think that in and of itself is worthy of an upvote – so don't be too surprised if some of the upvotes keep coming.
In the end, upvotes and downvotes are a tool each member uses at their own discretion. Individually, they can sting, they can frustrate, they can reprove, and they can encourage. Collectively, though, they are a mechanism which on the whole encourages people to submit quality work, and discourages people from submitting shoddy work, thereby maintaining an overall high quality for the site.
If I was aware of a chronic plagiarizer who consistently misrepresented copied-and-pasted passages as their own eloquence – perhaps inadvertently – I might start using my right to downvote (and comment) as a way to let others know what was going on, and to encourage better netiquette. But I'd also use my privilege to upvote as a way to acknowledge the pertinent research after the source was properly cited.
It would be interesting to see if those comments got upvotes as well; if so, that might indicate some measure of community support or appreciation for calling attention to the offense.
Lastly, if I was deciding to do this as a new course of action, I wouldn't barrage the user with a flurry of comments on Day 1. This can be fixed one question at a time, over time – that would be my approach. Gentleness is in order, I believe, even if plagiarism itself can be a grievous offense in some situations.