Theme Signup page
This is something we've been seeing for awhile - it's not a "phantom" site in the system, it's just the default wp signup page. When you go to a subdomain that doesn't exist, like "asdfasdf.nycga.net", wordpress tries to fix that by signing you up as that username.
wp-signup.php is in the root directory, so it bypasses the site-wide theming. I tried a few quick fixes for this last night but didn't make any progress (Theme My Login plugin just broke a bunch of stuff).
This looks like a promising solution but I couldn't get the htaccess stuff quite right:
You just bypass /wp-signup.php and instead go to /wp-content/themes/nycga/wp-signup.php, which we can style however we want.