Hi Gio,
According to my research, one helpful solution is to
turn on moderation for new users' posts. See this blog for details:
http://www.theopenhouseproject.com/2009/09/02/end-google-group-spam/
However I also learned that spammers have found ways to make their
emails appear to come from legitimate Google Groups members (ie, email
spoofing), so this may only be a temporary solution. But I haven't
found anything else, besides
move to another service as some groups have.
The fundamental problem is that Google decided not to use its spam
filter technology (the one that works incredibly well for Gmail) for
Google Groups. Maybe this will change in future...
hope that helps.
Isa