How to stop unauthorized spam into my yahoo groups-?

Try this: Open one of his posts and to the right of the message check ban member. or author. you could also find him in the members list click edit membership below his i.d. scroll down click ban member and follow the prompts. You could change your groups posting options so that all posts are moderated. as soon as someone tries to post an inappropriate message ban them. I see you removed him already so maybe I'm of no help.