What would be a good response to a student that committed plagiarism on an assignment?

Depending on the grade, either make them totally re-do the assignment, or deny the points for the copied part. Younger students are less likely to understand that what they did was wrong, so you should give them a chance to do the work over for a grade. Older students need to see that plagiarism is wrong and will not earn them any points - I always told mine that I gave the points to the person who wrote that material, but not to the one who copied it! You should be careful to make a difference between the behavior and the student - plagiarism is wrong, but the student isn't a bad person for trying it. It was a mistake, and they should be made to see that they can correct it and learn how to summarize and write things into their own words.