From The Chronicle forum A Plagiarist’s Tale Prof. Eumaios offers this:
In addition to threats, you might also want to try this trick. Snatch a couple of wretched essays from free-paper sites. (You can almost pick them at random.) Mark them up with all the savagery you can muster. If you try just a little, you can give a worse-than-average paper-mill essay a grade of less than zero. Photocopy the graded essays and give them to students. Tell the students that even if they manage to get away with cheating, they’ll still fail. Tell them that the paper mills can get away with selling drivel and dreck because the customer has no recourse; a student can’t say, “But Mister Eumaios, I paid good money for that paper, and you gave it a grade of -47.”