Once a case of plagiarism has been identified and fully documented, it is the instructor’s responsibility to take action. It is inappropriate to ignore cases of plagiarism under the misguided hope that the student will not commit the offense again. It has been my experience that students who have plagiarized another writer’s work (and escaped detection) will be unlikely to reform without some kind of intervention. Therefore, the sooner the instructor addresses the issue, the more likely the student is to correct this behavior.
When plagiarism has been detected, two factors often influence the instructor’s actions: the degree of the violation (i.e., how flagrant the violation was) and any previous occurrences for which the student was reprimanded. The student’s understanding of what constitutes plagiarism may also have some bearing on the instructor’s response, but this varies based on the knowledge the instructor expects the students to have and whether or not any work has been done in the class to define plagiarism.
In most circumstances, it is valuable to have the student respond to the charge before moving further. I provide the student with what I have found and ask the student to respond. I have seen responses that vary from indignation to contrition. Some students simply drop the class and I don't hear from them again.
In each case I use the opportunity as a teaching moment and stress the value of proper citations and the importance of original work. I also use the occasion to explain the impact of plagiarism on the original author, the class, and the student. In almost every case, students that respond acknowledge the infraction and vow not to let it happen again.