You can set them as Neutral, and they won't show up. However, you won't be able to do Campaign AI Scripts anymore. As far as actually removing a computer player from the lobby, I don't think you can. You can give them their own force, and put an Invisible color tag in front of the force name, but I don't believe there's a way to eliminate a computer player from showing up in the lobby, aside from making them a Neutral player instead.