ethics - How to deal with cheating when university policy is ineffective?

I am co-teaching a relatively large class with two other colleagues in a big university. Each semester, most students are amazing, trustworthy, and well-behaved and it is a pleasure to work with my students. But there is a very small group of students each semester who cheat and I don't know how to deal with them.

The university has a detailed, strict, and elaborate policy for dealing with cheating. We send students suspected of cheating to a special committee where they decide if the student actually did anything wrong. The committee demands a copious amount of evidence before it convicts any student, yet at the same time, the university policy is so restrictive that we cannot collect evidence. For example, we are not allowed to do anything that indicates or suggest a student may have cheated. We are not allowed to mark their papers, move them during exams, or even talk to them. This means that most students get away with cheating if they are sent to the committee. Students who have copied identical answers from a neighbor over and over in quizzes got away with it in the committee. This process is so pointless that the course coordinator does not report most of the cases to the committee anymore.

I don't mind that a few students get away with cheating but the universities' restrictive policy shuts all the doors to any further discussion with the students. I am not looking for ways to punish my students. I want them to understand why cheating is unfair to other students and, hence, unacceptable. I want them to understand integrity and honesty are far more valuable traits than getting good grades in college.

Any suggestion on how I can communicate these ideas and their importance to my students?

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Edit:

I am sorry for not being clear. My question is not about how to prevent cheating. It is bout how to have a conversation with cheaters.

We do what we can to prevent cheating. We randomly seat students, make two copies of the exams, assign a good portion the final grade to labs, and use various other techniques. This question is about reaching to the cheaters and having a discussion with them.