Whoops.  Forgot to say that Steve Havlich's idea 
about buffer-disable-undo also needs to be added to 
truly get rid of the unwanted output from whatever
kill ring its being pushed into when delete-region
is called:  

  
 >  > (defadvice fi::subprocess-filter (before restrict-acl-buffer-size act pre com)
 >  >   "Restrict the buffer size of the ACL buffer to about `max-acl-buffer-size'."
 >  >   (let ((buffer (process-buffer (ad-get-arg 0))))
 >  >     (cond ((and buffer (eq buffer (get-buffer fi:common-lisp-buffer-name)))
 >  >            (save-excursion
 >  >              (set-buffer buffer)
 		   (buffer-disable-undo) ;; added here *** 
 >  >              (if (> (point-max) max-acl-buffer-size)
 >  >                  (delete-region 
 >  >                   1 (- (point-max) 
 >  >                        (truncate (* 0.9 max-acl-buffer-size))))))))))

-dr