*scratch*.el

(require 'cl)

(defun is-magit-buffer (buffer)
  (let ((name (buffer-name buffer)))
    (and (= ?* (aref name 0))
         (not (string-match "^\\*magit\\*" name)))))

(defun kill-magit-buffers ()
  (interactive)
  (loop for buffer being the buffers
        do (and (is-magit-buffer buffer) (kill-buffer buffer))))
        
;; Plus something like this
(add-hook 'magit-quit-session 'kill-magit-buffers)

Might need some tuning.