I guess what you are looking for is a robots.txt entry like this:
User-agent: *
Disallow: /www.example2.com
Let's suggestion you have more than 100 'parked' exampleNR.com URLs, but don't want to write a line for every single one of them...use this:
User-agent: *
Disallow: /www.example*
The problem is, it actually is not officially supported, but many robots like Googlebot are able to understand those easy wildcards. RegEx are definitely not supported. for additonal information