Thanks for bringing this issue up. I've passed it on to the team. In
general, if the group had normal postings, you could use the "Report
this message" link (under "More Options") to report things like this.
In cases like this where there are no postings to report (but where
the group clearly abuses our terms of service), you can send an email
to abuse @ googlegroups.com providing the information that you have.

Thanks for your help!
John