Our exercise can't be cheated on because we discuss it with the candidate during the in-person interview and even ask them to change the code in a simple way.

Also, the exercise is designed to be original and so a solution can't be Googled.

We're talking only about the "in their own time" take-home types of tests. The type of cheating I'm talking about is outsourcing the problem or even just having a friend do it for them.