How to remove admin menu pages inserted by plugins?

You need to use the right hooks (which are not always the same as the URLs/slugs), and it doesn’t hurt to use a hook that runs later (e.g., admin_init):

add_action( 'admin_init', 'wpse_136058_remove_menu_pages' );

function wpse_136058_remove_menu_pages() {

    remove_menu_page( 'edit.php?post_type=acf' );
    remove_menu_page( 'wpcf7' );
}

You can use the following to debug:

add_action( 'admin_init', 'wpse_136058_debug_admin_menu' );

function wpse_136058_debug_admin_menu() {

    echo '<pre>' . print_r( $GLOBALS[ 'menu' ], TRUE) . '</pre>';
}

This gives (for my setup) the following for the Contact Form 7 plugin menu page:

[27] => Array
        (
            [0] => Formular
            [1] => wpcf7_read_contact_forms
            [2] => wpcf7
            [3] => Contact Form 7
            [4] => menu-top menu-icon-generic toplevel_page_wpcf7 menu-top-last
            [5] => toplevel_page_wpcf7
            [6] => none
        )

The array element with key 2 is what you are looking for: wpcf7.