The following is from Google Groups.
If they get enough feedback "click More Options at top of message and
select report message" they may see fit to block these messages.

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How can I report cases of Usenet abuse from people posting though
Google Groups?


To report cases of Usenet abuse, click the "More options" link at the
top of the message and select "Report this message." On the report
page, describe why you think the post is abusive.

Please be aware that, while we appreciate how annoying off-topic posts
can be, Google does not regularly monitor or censor postings sent to
Google Groups, and we aren't able to pursue most complaints that we
receive. However, we do try to prevent wide-scale spam and other forms
of Usenet abuse, most notably involving numerous repeat offenses or
the posting of someone's personal information (such as credit card or
social security numbers). We'll use all of the information we receive
regarding abuse and spam to make large-scale changes to fight all
types of abuse. In Google Groups discussion groups (not Usenet
newsgroups), you can also ask the owner or moderator to remove a post.

For more information, please click the link below to view our Terms of
Service.