bbPress.org

Do you know where I can modify the redirect page so that it only shows the 404 page not found message and not a list of my site’s pages

It usually depends on the theme, but 404.php is a good start. It probably has some function or functions that put out a sitemap, but that’s just a guess, and every sitemap type plugin is different.

In addition to bbPress, there are legitimate Membership plugins that are better equipped to handle things like private pages or members-only sections of websites. If you haven’t used any, Restrict Content Pro and Paid Memberships Pro are good ones.