Cheating was rampant in all the institutions where I've taught CS. On the first day of class, while reviewing the syllabus, I would point out the official university policy on cheating and plagiarism (by URL). Then I would say that if they were going to cheat, they'd have to do it well enough so that I couldn't detect it. Otherwise, immediate F on the offending assignment. It also helped that on programming assignments, I always supplied the API that they needed to implement. It changed from semester to semester and was unlike any other API that I could find on the web. I, too, have seen code copied from elsewhere with copyrights and explanatory comments intact.

My wife teaches also. She once had a student who handed in a mostly (i.e., 80%) plagiarized assignment. The student's excuse when confronted. She'd had a friend do the assignment for her, and it was that friend who was guilty of the plagiarism!

Ceci n'est pas un seing.