nobody may be intentionally doing this. I am a moderator for another group, in which the same thing is happening. It is automatically generated mails i suppose. In the yahoo moderator account there is an option to block the spam mesaage, also an option is available in your yahoo account too. Please enable those options.
We can maybe find out the culprit (which could well be a virus) ourselves... could you give me headers of the spam mails please...we can try to correlate the ip addresses to other mails in the list....