Removing them does not exist a page indicating that the page does not exist, I think it would be a good idea to create a custom error page and apply to .htaccess this code
It's not mandatory. Supposing the page is not found, the Apache server already reports "404" header to the browser or search crawler. Seeing that, search crawler will delete this page from it cache and it won't appear there again until it appears on the website (in most cases, it won't never appear).
ErrorDocument directive is mostly used for overriding default 404 pages. If you are happy with the default one, there is nothing to touch.
Additionally, let's not forget it won't work on all servers, but only there where it's allowed.