Potentially if there is a house associated to the FC they're using it for their own personal housing or may think to sell the FC in the future. More light hearted idea might be they just log in log out and don't pay attention to your messages, particularly if they haven't kicked you out already as that'd be a sign "it's theirs and they've no interested in sharing". As everyone said though that individual has done nothing against the rules, from what you've said so far at least, so there isn't a way that you can force them to give it back.