Redirect arbitrary sub-domains to custom 404 when wild card A record is set?

[anagio]

Is there a way to capture all requests to sub-domains which do not have
a virtual host setup, and redirect to a custom 404 page? I will have a
wild card A record setup *.example.com and all our users will have a
sub-domain username.example.com. If someone enters a sub-domain which
does not exist how can I redirect to a custom 404 page rather than have
it resolve since wild card is setup?

Thanks

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,218654,218654#msg-218654