how stop spam to a discussion group?

[Rudy]

same yahoo member sends spam to our discussion group
The spammer is NOT a member of our group...I am a group moderator, so I get his posts for moderating, and I reject his posts...over and over and over, ad nauseum.
He/She posts about 1 or 2 times a week, always pushing some educational school, or learning program, or degree program...can't get rid of this guy and Yahoo is NO help.