Can I block Chatter Free users from seeing specific posts?

Question

We want to bring additional users into our Chatter with Chatter Free licenses, but realized that some sensitive posts have been made outside of bespoke private groups. This is causing a problem since we don't want the new users to see those posts. I can't solve this with Visualforce and Chatter does not support blocking users.

Short of deleting dozens of posts, are there any ways around this? Can posts and their comments be moved into private groups after the fact?

Update

A quick test in the Developer Console shows that you cannot UPDATE FeedItem so these posts cannot be re-parented.

Answer 1

You could perhaps implement a 'Chatter Censoring' orchestration, where you write a Trigger on FeedItem and scan the post for certain (configurable) sensitive keywords. You can then either prevent posting of this post all together or automatically post it to a relevant private group.(set ParentId on the FeedItem)

    trigger FeedItemCensor on FeedItem (before insert) {
    for (FeedItem fItem : trigger.new){
       //you could retrieve offensive keywords from a Custom Setting
       if(UserInfo.getProfileId()=='PROFILE_ID' && fItem.body.indexOf('Offensive') > 0) 
       fItem.addError('You cannot post');
       //fItem.ParentId = '0F9XXXXXXX';  //Alternatively set group Id as parent Id
      }

    }

Consider having a Custom Setting where the Name=> Value pair is potentially Keyword => GroupId, so that you can scan for keyword and then quickly understand which Group to attach the post to.