If this young relative is under the recommended age you could ask Facebook to ban the user but thats a bit extreme.
I don't know your router but it should be able to block this relatives homepage.
If that doesn't work just block facebook totally with a timer(when said relative is home alone/unsupervised), this means no one during the set times can use it, But it may have the results you are looking for.
It may be time to assert that having a computer and internet access is a privilege and trust is an earned thing, not just a given.
But that is your own buisiness and this is not a parenting/family forum.
If none of those Ideas work, hopefully a networking guru will jump on this thread and give you the answers you need.
yea i think if anything the time limit settings on the router would work
but u cant block the homepage, the homepage (immediately after you log in) just reverts to 'facebook.com'
the only time u would get a unique ip is if u were to click on the profile tab and view ur own profile
and the whole 'computer usage is a privilege' thing was done before but thankfully the kid doesnt live here, lol
im just anticipating the possibility that summer vacation may result in the kid being alone in my house during the day, and i didnt want to restrict others (who would be getting home from work while i am still at work) from being able to use the computer once they get home