How do I stop all the spam being emailed to me as groupname-owner@yahoogroups.com?

Modigleana asked: I have two yahoo groups. Both groups’ spam filters are turned on but on a daily basis, I get spam sent that has been sent to groupname-owner@yahoogroups.com re-routed to my personal email address. These are not messages from members but are from people external to the group. They are flagged [...]