Is it possible to unset or override system messages with Rules?

Subscribing. I'd also like to be able to do this, specifically so that I can override the default message when a user saves changes to their account.

Yep please. Not only this message, but any message created for the same event we are working on with the Rule.

If that event has already a message and we set one within the Rule we will have both :(

Is there some configuration we are missing?

Seeking how to solve this issue, too...

I've found the related issue: https://www.drupal.org/node/409090
and an obsolete module https://www.drupal.org/project/rules_better_message
which could solve the problem by cleaning unnecessary messages and showing your own one.
But this module does not work for me in 7.x :(

You'd need a custom action to do this. This actually isn't that hard to do.

In YOUR_MODULE.rules.inc:

function YOUR_MODULE_rules_action_info() {

  return array(
     'clear_messages' => array(
        'label' => t('Clear any pending Drupal messages.'),
        'parameter' => array(
          'type' => array('type' => 'text', 
                                 'label' => t('Message Type'),
                                 'description' => t('A key like "status", "warning" or "error"')
                                ),
        ),
        'group' => t('YOUR MODULE'),
        'base' => 'YOUR_MODULE_action_clear_messages',
    ),
  );
}

function YOUR_MODULE_action_clear_messages($type) {
  //clear messages of $type.
  drupal_get_messages($type, TRUE);
}

I haven't tested this (been a while since I worked on Drupal 7; I'm doing D8 now), but this should be close to what you want.

Thank you, Rob. I solved my initial problem without Rules, though.
In fact, I needed a bit more - to show different status messages depending on whether a content was published or not. It's needed for forum topics only.

Here is my code:

/**
 * We customize 'Forum topic ... has been created.' message when
 * it goes to the approval queue. This is done in two steps:
 *
 * 1. Add new status messages implementing hook_node_insert().
 *    This is done for both Published and Unpublished nodes.
 *    For Published nodes we just duplicate the old message.
 *    For Unpublished nodes we inform that the message is waiting for approval.
 *
 * 2. Remove old status message implementing hook_form_FORM_ID_alter().
 *    We override 'submit' action where we remove old messages in case it
 *    matches the pattern '@type %title has been created.'.
 *    We only remove a single match, so even if we duplicate a message
 *    on the first step, one of them will still be displayed.
 *
 * We customize also the Drupal behavior when a Forum Topic was updated and
 * the new revision goes to the Approval Queue. Now it redirects a user to
 * the Forum Category and shows the similar status message to inform the user.
 * We use hook_node_update() for that.
 */
 
/**
 * Implements hook_node_insert()
 */
function MY_MODULE_node_insert($node) {
  /**
   * Add custom status messages when a node is inserted
   */
  if ($node->type == 'forum') {
    if ($node->status == NODE_NOT_PUBLISHED) {
      drupal_set_message(t('Your forum topic has been queued for review by site administrators and will be published after approval.'));
    }
    elseif ($node->status == NODE_PUBLISHED) {
      $message = t('@type %title has been created.', array('@type' => node_type_get_name($node), '%title' => $node->title));
      drupal_set_message($message, 'status', TRUE);
    }
  }
}
 
/**
 * Implements hook_node_update()
 */
function MY_MODULE_node_update($node) {
  /**
   * If a Forum Topic was updated and the new revision goes to the Approval Queue
   * we redirect a user to the Forum Category and show a message to keep him informed.
   */
  if ($node->type == 'forum') {
    if ($node->status == NODE_NOT_PUBLISHED) {
      drupal_set_message(t('Your forum topic has been queued for review by site administrators and will be published after approval.'));
      drupal_goto('forum/' . $node->forum_tid);
    }
  }
}
 
/**
 * Implements hook_form_FORM_ID_alter().
 * @see MY_MODULE_node_insert()
 */
function MY_MODULE_form_forum_node_form_alter(&$form, &$form_state, $form_id) {
  /**
   * Override 'submit' action for 'forum_node_form' form
   */
  $form['actions']['submit']['#submit'][] = '_MY_MODULE_form_forum_node_form_submit';
}
 
/**
 * @see MY_MODULE_form_forum_node_form_alter()
 */
function _MY_MODULE_form_forum_node_form_submit($form, &$form_state) {
  /**
   * Remove the old status message 'Forum topic ... has been created.'
   */
  if (!empty($_SESSION['messages']['status'])) {
    $node = $form_state['node'];
    $old_message = t('@type %title has been created.', array('@type' => node_type_get_name($node), '%title' => $node->title));
    $old_message_key = array_search($old_message, $_SESSION['messages']['status']);
    
    if ($old_message_key !== FALSE) {
      unset($_SESSION['messages']['status'][$old_message_key]);
      
      // Reset array indexes. Otherwise it doesn’t work with some theme templates.
      $_SESSION['messages']['status'] = array_values($_SESSION['messages']['status']);
      
      // Remove the empty status message wrapper if no other messages have been set.
      if (empty($_SESSION['messages']['status'])) {
        unset($_SESSION['messages']['status']);
      }
    }
  }
}

Ref (rus): www.ishmuradov.com/page/drupal-7-izmenjaem-statusnye-soobshhenija

Hope this helps someone,
Alexander

Here is code for a Rules action to clear all or some messages from the site (specified by type and/or regular expression applied to the message text):

function mymodule_rules_action_info() {

  $actions['mymodule_rules_action_clear_messages'] = array(
    'label' => t('Clear system messages'),
    'parameter' => array(
      'type' => array(
        'type' => 'token',
        'label' => t('Clear messages of type'),
        'options list' => 'mymodule_message_types',
        'default value' => 'all',
        'optional' => FALSE,
      ),
      'pattern' => array(
        'type' => 'text',
        'label' => t('Regular Expression'),
        'description' => t('If specified, messages of the type specified above will only be cleared if their text matches this pattern.'),
        'optional' => TRUE,
      ),
    ),
    'group' => t('System'),
  );

  return $actions;
}

function mymodule_rules_action_clear_messages($type, $pattern) {
  if ($type == "all") {
    $message_type = NULL;
  }
  else {
    $message_type = $type;
  }

  // Clear all messages of specified type.
  $all_messages = drupal_get_messages($message_type);

  // If no RE provided, done.
  if (empty($all_messages) | !$pattern | $pattern == "") {
    return;
  }

  // Reset messages that do not match RE.
  foreach ($all_messages as $category => $messages) {
    foreach ($messages as $message) {
      if (preg_match($pattern, $message) != 1) {
        drupal_set_message($message, $category);
      }
    }
  }
}

function mymodule_message_types() {
  return array(
    'all' => t('All'),
    'status' => t('Status'),
    'warning' => t('Warning'),
    'error' => t('Error'),
  );
}

This seems to have been answered several times already.

Many thanks for the code in #8. It worked perfectly!