They don't even understand that it's cheating....

We've given them so much group work for so long that they just give me dumb looks when I get on to them for copying homework. I find that I have to be very specific that I want *their* work and their work only on certain quizzes and assignments. I had assigned a crossword puzzle for a fun vocab activity for my Physical Science students. I was standing right over a kid a he copied his neighbor's work. He looked at me and said "WHAT?!?" I love group work, but I need to find a way to teach the difference between group and individual work.