gnus-spam-process-newsgroups

gnus-spam-process-newsgroups is a variable defined in `gnus.el'.
Its value is

nil

Documentation:
*Groups in which to automatically process spam or ham articles with
a backend on summary exit. If non-nil, this should be a list of group
name regexps that should match all groups in which to do automatic
spam processing, associated with the appropriate processor.

You can customize this variable.