In the interest of avoiding violence, there really should be some process for secession. I'd want to require at least two consecutive affirmative referenda, say, two years apart, with a threshold of 2/3 of eligible voters (i.e. absolutely everyone above 18). When the first referendum comes back in favor of secession, the state's Congressional delegation immediately becomes non-voting members and the state loses its electoral votes, essentially downgrading it to territorial status. It also cannot receive any federal expenditures during the following two years (not even as foreign aid), and of course no taxes would be collected from it either.