So, the newsgroup you read is suddenly filled with articles consisting of nonsense...
  (apart from the local kooks)
.
One or more induhviduals are running a bot called newsagent with the sole purpose of disrupting Usenet.

The from: and nym associated with the post is always a forgery based on headers collected from news.admin.net-abuse.usenet and news.admin.net-abuse.email.  The person you think sent the message actually didn't.

In any case, a quick read of the headers makes this abundantly clear.
 

Now, what can you do to stem this flood?

1. Complain to your ISP and firmly request that they install filters.
(Both supernews, and Onramp [now owned by Verio] use filters)
Cleanfeed is one of the free filters available to ISPs

2. Don't discuss it much in public, beyond the simplest guidance to help people that need it.
Reason being we don't want to help the deveoper of this abuseware improve it.

How do I keep this stuff off my system?

1) Windows systems:
  - NFilter  http://www.nfilter.org
    NFilter provides a method for filtering your Usenet feed
    and while fairly robust still has limitations.

    To eliminate crossposts between your group and nanae/u
    try the following line.

    your.news.group    drop   xref:news.admin.*

that will effectively drop all articles crossposted to the admin groups.

2) _NUX
  -  Little help here .. :)

  Please post suggestions to news.admin.net-abuse.usenet Attn: LXIX