I like how people responsible for .pl (Poland) domain handle this. First of all, they list only very basic data, with no names, addresses, etc. when you query their whois. To see the full data, you have to go to their website and type the captcha, which filters out at least some of the bots. But even there they display the data if it belongs to a company, they won't show any details if it's registered to a private person.