Yahoo! Groups offers great Spam Filtering to help moderators block spam posts to the group.

If you’re receiving unwanted messages or spam, you may want to unsubscribe from the Group.

While no email-based group is completely safe from potential spam abuse, Yahoo! Groups reacts quickly to any complaints of spam.

Yahoo! Groups members may not use our service to send unsolicited or unauthorized advertising, promotional materials, spam, chain letters, multilevel/pyramid schemes, or any other form of solicitation. Yahoo! reserves the right to remove any member who violates our Terms of Service.