There is no reason to leave him permitted to your conference, specifically. To depermit him, go into the Conference Manager with the link at the top of your conference page. Click on the Change Config. David is now no longer permitted to participate in your conference. It's also not a bad idea to create a topic where the participants can make suggestions on additional topic areas or other improvements to the conference.
Topics often evolve as the conference progresses.
COW allows you to configure individual topics to be private separately from your conference. Your conference may be public, but certain topics could be private. Or your conference could be private and certain topics even more so. A realistic reason for making a topic private is, for example, if you wanted an arena where only the Professor, TA's, and graders could post and read messages. However, the private topic in the private conference will be seen only by some subset of that conference participants.