BuddyPress.org

Boy, other than coding a special function that checks to see is a user has filled in all required profile fields upon log in, I’m not sure at this point. What does happen in this scenario though is that the next time a user edits their profile data, they will not be able to save any changes unless they fill in the required fields.

You could ask them to update their profile data, but that is always a hit or miss proposition.