To reduce junk messages coming to Yahoo! Mail by way of its Yahoo! Messenger part, you can set it up to accept IM conversations only from people (and other senders) in your address book.
Block Unknown People from Yahoo! Messenger in Yahoo! Mail
To prevent people (and advertising or even scamming robots) not in your contacts from sending you instant messages using Yahoo! Meseger in Yahoo! Mail:
- Click Options in the top Yahoo! Mail navigation bar.
- Select Messenger Options from the menu that comes up below.
- Go to the Privacy category.
- Make sure Block all users not in my Contact List is selected under Block Users.
- Click OK.
(Updated October 2012)