Gnus remove group

After years of using Gnus for my mail I had some anomolous groups like “mail.sent” (later I had reworked this to archive.sent, but couldn’t get rid of mail.sent); I had tried deleting the folder on my hard drive, but this only resulted in errors with trying to enter the group from the group list, but still the group was shown (even with a number of unread messages). Turns out the answer is easy:

G <del>

Now those groups are gone, after years. Hooray!