Dynamically add/remove post viewers that aren't controlled by a group

Just to clarify, you are posting the the news feed and adding individual users as viewers/participants, and want to adjust the people who can see a post dynamically. E.g., some time during the day, you want to be able to decide that a user no longer has permissions to see a post. Is that correct?

AFAIK Once the News feed posting is done with the established viewers, you may not change the viewers list to that post. However you may do this dynamically viewing the post by reposting the feed again with the new viewers. Lets say for eg the post A has been posted for the first time with viewers A and B, Now after the changes the same post has to be viewed by Viewer B and C then post the same Feed A again with viewers as B and C so that B can see both Posts and C can see the second Post and A cannot see the new post, Hope this might help!!!