Make your expectations clear and follow through
This sales pitch from the online paper mill called "Due Now" is a good example of what you're up against:
“Are you sick-and-tired of spending hours in a library or online searching for an essay on your topic—only to end up with no essay and no quality information? Tired of slaving all night long writing an essay—only to get a garbage grade? Do you have better things do with your time than spend it writing a useless essay?”
It’s up to you to stress the importance & benefit of original work to your students. To remind them that university is not just about grades, it's about learning.
The best prevention involves talking about the problem. Educate your students about academic integrity and what plagiarism is and isn’t. Bring examples. Explain why citing is important. Explain how to paraphrase and cite properly. Ask a librarian to come and talk to your class.
Ask for a "Student Awareness Statement" to affirm the work is original, has not been submitted for academic recognition or credit in any other class and has not been copied from other sources. The statements are signed and dated. You may choose to distribute the statement at the beginning of every term or with each assignment. Download some examples of previously used student awareness statements [PDF - 124 kB].
The University of Maryland has an honour pledge that is meant to be handwritten by students and reads: "I pledge on my honor that I have not given or received any unauthorized assistance on this assignment/examination." Writing out the code makes students think about what it says every time they commit it to paper.
You should also talk about what happens if students are caught plagiarizing, collaborating without authorization, cheating on exams or sharing information. Let students know there’s a discipline process in place and the consequences can be serious.
From visiting online paper milll websites to using citations in your lectures, read about more ways you can encourage academic integrity in your students.