How to deal with cheating when university policy is ineffective?
Since you speak of "a small group who cheat," it sounds like most of the students do understand the policies and importance of honesty.  That remaining small group who cheats?  Nothing that you do or say is going to convince them not to cheat.

Likewise, university policies often make it hard to actually punish cheaters, because they begin with the presumption of innocence, which is generally a good idea.

A better way to approach the problem is to adopt a "layered defense" strategy of a number of small actions that reduce the cost/benefit ratio available from cheating.  Some of the sorts of strategies that I have seen include:


Increasing the separation between desks
Randomized assigned seating, which is likely to break up cheating conspiracies
Printing less questions per page, with lots of white-space for working the problems, so that an answer will not be visible for as long a period of time
Problems that are based on "showing your work" rather than obtaining the right answer.
Problems with many possible solutions, that convert copying into plagiarism.
Putting more credit into projects and labs, rather than exams.


None of these alone will solve the problem, but the more of them you can enact, the harder it is for cheaters to prosper.  Some of them may even help the non-cheating students, by forcing them to engage more deeply with the material.  The balance, of course, is that it all takes time and effort from the instructors and TAs, so you need to make your own cost/benefit analysis as well.