Same nothing that prevents a person from copying it onto a USB stick before they leave.
However, that's not GitHub's problem. GitHub is "told" (by removing the person's access from the repo) that they shouldn't have access to the repository or its code. It is within GitHub's ability to remove access to their fork and remove it.
However, that's not GitHub's problem. GitHub is "told" (by removing the person's access from the repo) that they shouldn't have access to the repository or its code. It is within GitHub's ability to remove access to their fork and remove it.