In the same way that guests must be approved by the Frame owner to send photos to each Frame, the Frame owner may revoke this permission at any time.
The full list of Users associated with each Frame can be viewed on the Frame under Setting -> Manage User. Tapping on the name of the user will allow you to view all photos from that User, Delete (Block) that User or Delete that User and the full set of Photos that they have sent to the Frame.
