There are some legitimate uses of Turnitin.com besides as a plagiarism checker. (Though that is useful and I wish I had told my students to turn in their annotations that way. Big plagiarism fail on 25% of the grade.)
Have the students turn in their work ahead of time and check the reports. Then they have a clear indication of plagiarism, if it was accidental, and can fix it. If it was deliberate, then they know they will get caught and have time to change.