As aheilbut stated you can use .htaccess to rewrite the destination if the request matches /signup. This question is highly dependent on your backend. If you're using a framework that allows you to explicitly define the URLs in your app then you can just comment that line out. You could also try triggering a 3xx to redirect them to the homepage upon requesting that page.
Really though, if your site is pre-alpha and if you don't want people to be on it to begin with you should have your entire site blocked by at least basic authentication. http://httpd.apache.org/docs/2.0/programs/htpasswd.html
Any host worth paying will have some kind of option for "password protecting a directory."