Remove a menu item created by a plugin

function remove_submenu() {

    remove_submenu_page( 'edit.php', 'edit-tags.php?taxonomy=faq-topic&post_type=question' );
    remove_submenu_page( 'edit.php', 'edit-tags.php?taxonomy=faq-tags&post_type=question' );
}

add_action( 'admin_menu', 'remove_submenu', 999 );

Please read the Codex. remove_submenu_page() need two parameters and the right hook.

And very important: Use a very, very, very high priority in your hook! If you use a low priority, your function will be executed before the menus will be added. So there is no menu to remove. If you use a high priority, there is a good chance that your function will be executed after the menus was added.

This could be the tricky part.

UPDATE

After installing and inspecting the plugin, I found the solution. There are several issues and a few tricky parts.

The submenus are not added with add_submenu_page(), they are added with a custom post type. A simple search by add_submenu_page(), copy the menu slugs and removing the menus have to fail. I have to search for the cpt slug and use it.

After global $submenu; var_dump( $submenu ); I get this output

[more elements]
    'edit.php?post_type=question' => 
        array (size=7)
          5 => 
            array (size=3)
              0 => string 'FAQs' (length=4)
              1 => string 'edit_posts' (length=10)
              2 => string 'edit.php?post_type=question' (length=27)
          10 => 
            array (size=3)
              0 => string 'Neue FAQ' (length=8)
              1 => string 'edit_posts' (length=10)
              2 => string 'post-new.php?post_type=question' (length=31)
          15 => 
            array (size=3)
              0 => string 'FAQ Titel' (length=9)
              1 => string 'manage_categories' (length=17)
              2 => string 'edit-tags.php?taxonomy=faq-topic&post_type=question' (length=55)
          16 => 
            array (size=3)
              0 => string 'FAQ Tags' (length=8)
              1 => string 'manage_categories' (length=17)
              2 => string 'edit-tags.php?taxonomy=faq-tags&post_type=question' (length=54)
[ more elements ]

Now it was easy to remove the submenus with edit.php?post_type=question as menu slug and edit-tags.php?taxonomy=faq-topic&post_type=question / edit-tags.php?taxonomy=faq-tags&post_type=question as submenu slug.

If you watch carefully, the ampersand (&) is a html entity. It’s not possible just to copy the url part and insert it. So you cannot remove a submenu page with a un-encoded url, it have to be url-encoded.

And here is the final code:

add_action( 'admin_menu', 'remove_faq_subpages', 999 );

function remove_faq_subpages() {

    $ptype="question";
    remove_submenu_page( "edit.php?post_type={$ptype}", "edit-tags.php?taxonomy=faq-tags&post_type={$ptype}" );
    remove_submenu_page( "edit.php?post_type={$ptype}", "edit-tags.php?taxonomy=faq-topics&post_type={$ptype}" );

}