The border is already closed (at least around here) to "non-essential travel". So, no crossing the border for vacationing. But "full" closure would mean block

The border is already closed (at least around here) to "non-essential travel". So, no crossing the border for vacationing. But "full" closure would mean blocking "essential" trade (food, oil/fuel, ...), and blocking "essential" workers that commute across the border. It could technically be done, but it'd be incredibly disruptive. Even with full legal closure, the borders are porous enough for a border wall to have been a talking point in some previous election debates. Build a mile-tall wall out of mithril, and you'd still be left with a ton of unsecured coastline, connected by some rather convenient oceans...