Drupal - Disable editing of a node after it has been published

Yes, you can define dynamic rules in a custom module. See hook_node_access

Here's a sample implementation:

/**
 * Implements hook_node_access().
 */
function MYMODULE_node_access($node, $op, $account) {
  if (
    // The $node argument can be either a $node object or a machine name of
    // node's content type. It is called multiple times during a page load
    // so it is enough if you perform the check once you get the object.
    is_object($node) && $node->type == 'story' &&
    // Operation on which you want to act: "create", "delete", "update", "view".
    $op == 'update'
  ) {
    // Check if the node is published.
    if ($node->field_published[LANGUAGE_NONE][0]['value'] == 1) {
      return NODE_ACCESS_DENY;
    }
  }
}