Banning a user because he was using black hat seo

Hi all

One my users was using black hat seo and i banned the user. He had his own website domain mapped, and was using the supporter plugin. My question is if google blacklists a particular website, does the entire network get affected because all the sites would be under one ip? I was lucky to spot this because our network is small at the moment. When the network does get larger, it would be very difficult to spot this kind of users.

thanks
nehal