Node registration form block - hide if registered
Node registration not providing a function in it's API for that, however you could include the "node_registration\includes\node_registration.api.inc" file and use their internal function that the module use to check for user access for  a specific node.

// Include the file
    module_load_include('inc', 'node_registration', 'includes/node_registration.api');


 if (_node_registration_user_registered($node, $account)) {
    //do something
  }