It appears that all you can do is either transfer a feed or delete the feed. Anything else you'll need to do using permissions.
From the object reference:
FeedItem represents an entry in the feed, such as changes in a record feed, including text posts, link posts, and content posts.
This object is available in API version 21.0 and later. This object replaces FeedPost.
create(), delete(), describeSObjects(), getDeleted(), getUpdated(), query(), retrieve(), search()
Special Access Rules
You can delete all feed items you created. To delete feed items you didn’t create, you must have one of these permissions:
- “Modify All Data”
- “Moderate Chatter”
“Manage Unlisted Groups”
Only users with this permission can delete items in unlisted groups.
- If you use an Apex trigger to modify the Body of a FeedItem object, all mentions hyperlinks are converted to plain text and the mentioned users don't get email notifications.