Many people don't understand the extreme danger with a RC aircraft. I would have kindly told him the dangers associated with flying, and politly ask him to stand behind the orange line just in case. Once I landed, I would have gone further in depth about the dangers if they were still around. I think you did the right thing. After all, if something was to happen to him or his family, you would be liable. If you need to get stern for them to listen, then thats what needs to be done.