You threaten to ban any existing contributors lying about their code and you increase scrutiny of new contributors. You can go a step further and reach out to a

You threaten to ban any existing contributors lying about their code and you increase scrutiny of new contributors. You can go a step further and reach out to any other projects that the lying contributor works on and let their leadership know about the lying. There's plenty of ways to use social pressure that you're pretending don't exist. Not everything is a code problem. Some things are people problems and we've got about half a million years of experience with that.