you'd need to contact the web site host and report it there, what some people do is RSS feed group posts to their web sites. remove that member from your group. It is a TOS violation to
http://groups.yahoo.com/local/guidelines…
11) You cannot re-post or re-transmit content that belongs to another user without that user's permission. A Groups owner or moderator (or any other user) cannot re-post or re-transmit Groups content to any other site unless the person has the explicit permission of every group member whose content is being re-posted or re-transmitted.
post the above and see if it shows up on that site. to figure out who the guy is, scroll to the first post posted from your group to that site, and match it to the date of a new member- if posts are going to the site as they're posted- as you delete a possible member, send a test post and go check the site. if it posted, you have to keep removing members until you find where a test post