!("user" in user_groups) & article_namespace = 3
The first line checks if the visitor is logged in, and the second checks if they are editing a User Talk page. I got the namespace number from Manual:Namespace.
I made a special warning message, and then if they try again it simply disallows it. I am going to see how many hits it gets, before blocking the people/bots that trigger it, to make sure normal folks don’t get caught by it.