I think my professor kicked someone out 3 out of the 5 semesters I helped proctor. It's difficult to tell what someone is looking at, but occasionally you see someone really linger looking over at their neighbor and letting their vision wander to the same location over and over. I'm happy I never had to make the call on what to do, I just pointed it out to my professor. One of the professors went up and ripped a student's exam out of his hands twice. I don't know if they were allowed to retake it. The other professor was much nicer and just asked the student to move to the back of the room, away from other students to finish their test. She made sure to check his exam score against his previous performance in the course.