Put a 301 redirect in the .htaccess file, thus:
Redirect 301 / http://address.of.other.site.here
Alternatively if your webserver doesn't support or isn't configured to support .htaccess (not sure what you're running) you could use a meta refresh on the index page:
<meta http-equiv="refresh" content="10; url=http://address.of.other.site/">
you can then have a friendly message in there saying the site is being redesigned, and that they'll be redirected.