Hi
This problem is often discussed on here, it's a difficult one. If I were you I would try again, perhaps giving them as much proof as you can as to why you think they are wrong, and saying that you are only telling them as you hate to think that they are wasting time following the wrong family, and that you would be very pleased if someone did the same for you. Once you have done that, if they still ignore it, there isn't a lot you can do. If they are researching for a friend it may be that they have already given them information and now don't want to admit that they were wrong.
If they have a tree on Ancestry there is the opportunity for you to add a comment, you could say on there that it is the wrong family so that others see it.
Jan