Removing it from the system altogether would be tricky, but the easy way that would take care of all cases where people aren't actively trying to get around your limitations is to replace the view input/access with one that strips out the private option.

Beyond that you could hook into 'create', 'all' event and do a check

if($object->access_id == ACCESS_PRIVATE){

  // take some action, like resetting to default or forcing the user to go back and fix it

}

You can easily override the input/access view in your theme