Group Admins should turn off the ability for members to broadcast mail to all other members. I don't think that option should even exist. It opens the door for spammers as we've already seen. It also holds that door open for people who haven't figured out how to correctly post messages to the group board and instead send them to everyone. Already gotten a few of those misdirected messages.
Group members have the simple option to not receive e-mails from their group telling them that something new has been posted. But that won't stop something coming from a member who chooses to broadcast to everyone.
If someone has something to say to the group, post it on the group discussion page.