gnus-ignored-newsgroups

gnus-ignored-newsgroups is a variable defined in `gnus-start.el'.
Its value is

"^to\\.\\|^[0-9. 	]+\\( \\|$\\)\\|^[\"][\"#'()]"

Documentation:
*A regexp to match uninteresting newsgroups in the active file.
Any lines in the active file matching this regular expression are
removed from the newsgroup list before anything else is done to it,
thus making them effectively non-existent.

You can customize this variable.