Get Facebook to take down a group you find to be offensive or objectionable, even if the group doesn't have an administrator associated with it.

Navigate to the profile of any administrator of the group after you log in to your Facebook account. If you aren't sure of the group admin's name, ask a friend who isn't banned from the group to ...

As an extension to these two features, WhatsApp has now begun testing a new feature wherein a group admin can prevent users from sending "frequently forwarded messages".