The Action_RemoveView post message is currently available for all sessions. I think it would make sense to enforce possible access limitations to sessions, so that e.g. guest users / read only users cannot remove others from the editing document. I could not find anything related in the WOPI specs, so I would propose we add a custom entry to the CheckFileInfo: UserCanModerate: A Boolean value that indicates that the user has permission to remove other users from the editing session
This has been discussed, thanks for filing it, Julius!