Thanks to Nicolas for helping out on this one:
$(document).on('knack-view-render.view_xx', function (event, view, data) {$('#view_xx-field_xxx').datepicker('option', { maxDate: 0 });
});
This will make the user unable to pick a date in the future on the form. Just replace your view and field where the XX’s are.
To restrict to only dates in the future, simply change maxDate: 0 to minDate: 0