Don't assume you have been hacked. More likely you have contracted a virus or trojan through access to some site or file. These programs then use your email address book to send out these messages, changing or cancelling an email name/account will not help. You need to run a scan on your machine to detect these and remove them. There are many free programs on the net to do so. Afterwards get a better Antivirus program. AVG free or even better use Microsoft Security Essentials (also free) available on the MS website.
From: Dana Brown <firstname.lastname@example.org>
To: RIS-BOD@yahoogroups.com; TBIS-BOD@yahoogroups.com; email@example.com; iris <firstname.lastname@example.org>
Sent: Thursday, December 29, 2011 12:00 PM
Subject: [iris-photos] Fw: Strange email
Please don't click on any links that seem strange. I am afraid that my email has been hacked, just like Tom's was. As far as I know there's not much that can be done about it. Tom is still having problems and he closed that email account months ago!
Sorry for the inconvenience
Malevil Iris Gardens & Kennels
I own a dog and I vote!!