Should I scold the act of cheating before an exam?
Treat ethics seriously. Students should be aware that their actions do have consequences, and should be held responsible for their deeds and misdeeds. Jokes are inappropriate, and references about "cleaning your soles" with cheating-tainted diplomas are probably a little too hyperbolic.

That said, you can certainly give a warning to your students not to cheat on the exam. In some places, you must give such a warning as part of the exam instructions!