How to properly delete a nnmaildir folder?

[Jin Huang]

Hi,

Part of my configuration of gnus is below:

(setq gnus-secondary-select-methods
      '((nnmaildir "email" (directory "~/.gnus.d/mail/"))))

The group buffer list all the subdirectories well.  However, after I
removed one useless subdirectory, the group buffer still shows it, but
with a star before it:

       *: nnmaildir+email: xxx

I have tried to use G <DEL> to delete it from the group, it tells me:

  Couldn't delete group nnmaildir+email: xxx

What should I do to resolve this problem?

Best,

Jin