gnus-spam-newsgroup-contents

gnus-spam-newsgroup-contents is a variable defined in `gnus.el'.
Its value is

nil

Documentation:
*Group classification (spam, ham, or neither). Only
meaningful when spam.el is loaded. If non-nil, this should be a
list of group name regexps associated with a classification for
each one. In spam groups, new articles are marked as spam on
summary entry. There is other behavior associated with ham and
no classification when spam.el is loaded - see the manual.

You can customize this variable.