You can also use this code :

function block_wp_admin() {
    if ( is_admin() && ! current_user_can( 'administrator' ) && ! ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {
        wp_safe_redirect( home_url() );
        exit;
    }
}
add_action( 'admin_init', 'block_wp_admin' );