We recently removed some of the freeloaders from CommunityGeek.
These were a small group of people who joined via a VIP pass (they didn't pay) and occasionally lurked on the site without ever participating.
They didn't offer much to the community. We asked them to share their knowledge/experiences with the group. When they still didn't participate, we removed them.
Some will argue lurkers are valuable members. They might advocate on behalf of the community. Share knowledge with others they meet. Bring more people in. However, there's no evidence this was happening.
Once we established the notion of reciprocity within the group (i.e. you share what you're learing frequently in the course of your work) activity immediately increased. We can clearly see a measurable benefit to the entire community of removing those few members (and explaining why).
We established a clear expectation. We expect people to share what they are doing, thinking, and learning with others. A community can't survive if the lurker count drifts too high. We put a stop to it early.