150: Why can't I kill in Gnus based  on the Newsgroups/Keywords/Control
     headers?

  Gnus will complain that the "Newsgroups:", "Keywords:", and "Control:"
  headers are "Unknown header" fields.

  For the "Newsgroups:" header, there is an easy workaround: kill on the
  "Xref" header instead, which will be present on any cross-posted article
  (as long as your site carries the cross-post group).

  If you really want to kill on one of these headers, you can do it like
  this:

    (gnus-kill nil "^Newsgroups: .*\\(bad\\.group\\|worse\\.group\\)")