Hi Spike,
If you enable selective publishing then you have to explicitly publish individual posts using the Manage Publishing link, or if you also enable In Place Selective Publishing then as an editor you will see all the posts in the feed in place and can choose whether to publish them from there while non editors only see the posts you have published. The green check means a post is published and clicking it un-publishes, and clicking a green plus is what publishes it.
Though if you change to enable selective publishing after you already added feeds then the current posts are already published, so to remove posts you would have to un-publish them.
If you think it is not working correctly please try to reproduce the problem on the demo site so I can see it.
Hope that helps,
Joe