I discovered the Hikari Hooks Troubleshooter (http://hikari.ws/hooks-troubleshooter/) this weekend to find what plugins were loading my header with competing open graph metadata.
Enable it, and visit the frontend. There will be a modal dialog over your site which should list the hooks into wp_logout. One of these will be the offending plugin.
If you're still using the other parts of the plugin and there are no settings to turn that feature off, you should be able to remove it by adding
remove_action('wp_logout', $plugin_function) to your theme's functions.php.
Make sure you disable HHT when you're done with it.