Print out the headers of the email and do the following:

1. If possible determine where the mail originated from - everyone has to connect somehow (either dial-up, dsl, ect.). If you can identify a service provider notify them and get the account terminated.

2. If no service provider (you can assume the yahoo account is a "trash" account since it will be terminated anyway by yahoo) - then see if the email refers you to a website, or another email address. If so, then see who hosts the website by doing a 'whois' on networksolutions or whatever. If they are hosted by "Bob's hosting company" or something - notify them and get their site shutdown.

3. If it refers to a physical address or phone number, sign them up for every kind of junk mail offer you can find.