Blocking news groups from being fed to downstream servers is controlled by the configuration file /var/news/config/newsfeeds (see newsfeeds(4) for an extensive description).
The format for newsgroup blocking is:
server-name:sendgroup,!blockgroup1,!blockgroup2...::
For example, the entries
ME:*,!control,!junk,!local.*:: news.otherplace.com:*,!alt.binaries.*,!local.*::
Send all news groups except control, junk, and any group beginning local. to all servers by default.
Send all groups except groups begining with alt.binaries. or local. to the server named news.otherplace.com.
After you make changes to /var/news/config/newsfeeds, you must stop then start the news server using snsnews.