archiving mail with nnmail-expiry-target

[address@hidden]

In my .gnus I have the following

(setq nnmail-expiry-wait 'immediate)
(setq nnmail-expiry-target
        '(lambda (group)
         (concat "nnfolder:"
                 (format-time-string "%b-%Y." (current-time))
                 group)))


Is there any way to prevent gnus from opening the group to be archive
to in a buffer ?
I.E. when I go to exit emacs, I get prompted to save the Jun-2006.Inbox
buffer