Unwanted photos on My Photo Stream
Apple says:


  Add and remove subscribers to a shared photo stream
  
  
  Select iCloud in the Source list.
  Select the shared photo stream whose subscribers you want to change, and click the Info button in the toolbar.
  In the Subscribers section of the Info pane, do one of the following:
  
  
  To invite a new subscriber: Type the invitee’s email address.
  If the person you’re inviting doesn’t have an iCloud account, select the Public Website checkbox.
  To remove a subscriber: Select the subscriber’s email address, and press Delete.
  
  
  
  If you invited a new person to view your shared photo stream, an email and a notification are sent, asking that person to subscribe to your shared photo stream. If you removed an existing subscriber from your shared photo stream, the shared stream is immediately removed from the subscriber’s Apple devices. If you want to reinvite a subscriber to view your shared photo stream, delete the person’s email address from the “Shared with” section, and add the email address again.