I mean it would be enough to tell it to "Not cheat" or "Don't engage in unethical behaviour" or "Play by the rules". I think LLMs understand very well what you mean with these broad categories.
Not just negation demands, but also generally other tricks we use for thinking and communication shorthands. "Unethical behavior" here for example, we know what that means since the context is clear, but to LLMs that context can be unclear in which the unethical behavior can mean well... anything.