If you view this group at Google groups, go to a post by M15, and check the profile, you find:
"This account has been banned because it violated the Google Groups Terms Of Use."
Of course the spammer may well open another account somewhere else. Nearly all of the NGs I read have the same numerous M15 posts as in this group.
If you have a domain these days, it is easy to set up even hundreds of email accounts. You can use any of these to establish an account with Google or many other providers of access to Usenet groups. The host of my domains does not tolerate spamming. Thus, if Google contacted the host of the domain they could get the whole domain shut down. Some domain hosts do not care. In that case, Google likely would contact the host of the domain. If they did not correct the situation, then Google could block everything coming from all domains of that host until they correct the situation. However there are even more ways to allow posting at Usenet if someone is determined to do so and knows enough.