Filter the output of the Series post list to exclude posts the current user cannot access.

hide_noaccess_posts_pmpro_series.php

<?php
/*
 * Filter the output of the Series post list to exclude posts the current user cannot access.
 */

function hide_noaccess_posts_pmpro_series( $post_list_posts, $series ) {
	if ( function_exists( 'pmpro_has_membership_access' ) ) {
		foreach ( $post_list_posts as $key => $sp ) {
			$hasaccess = pmpro_has_membership_access( $sp->id, NULL, false );
			if ( empty( $hasaccess ) ) {
				unset( $post_list_posts[ $key ] );
			}
		}
	}
	return $post_list_posts;
}
add_filter( 'pmpro_series_post_list_posts', 'hide_noaccess_posts_pmpro_series', 10, 2 );