Well I think that's just a difference in wording of our statements.
No we couldn't force Modric and his family to join a team playing in the Mongolian Steppe or in Baghdad. We were able however to reject his strange demand that he could only ever play for Chelsea and stay in London, only for him and his family to have a change of heart over a season and realise that Madrid and Real actually weren't too bad for his career or lifestyle either.
So no, Southampton can't force Van Dijk to go somewhere he doesn't want to. I imagine his request that the only club he can join though is Liverpool is nonsense. And that he'd be able to make his peace playing for Man City or Chelsea. Or if not, Southampton can tell him that he'll be staying in Southampton next season instead?
As a side note, what is this recent obsession with equating football players with slaves? Its complete nonsense and almost borderline offensive to suggest these players, with incredible salaries and rights as opposed to the majority of normal workers, are slaves in any way (not just referring to your comment).