Blocking chat invitations from people not in your contacts list
This article describes if it is possible to block chat invitations from people who are not on your contact list.
Some Yahoo! Mail users may still see the option Block all users not in my Contact List within their Mail Options, however, this feature is no longer available for use.
In order to block the chat user from sending you further messages, you must first click Decline on the invitation, and then click Block to block the user.