You block them from using AI by making sure they know your project doesn’t want contributions from people using AI. Either they’re a decent human being and they

You block them from using AI by making sure they know your project doesn’t want contributions from people using AI. Either they’re a decent human being and they’ll comply with the project’s wishes, or they’re a sociopath who will violate the explicit request of the project and lie about the origin of their contribution and hopefully slip up and get caught doing so.