gtk:window-destroy (window)

window -- a gtk:window widget to destroy

Drop the internal reference GTK holds on toplevel windows. Signal handler for a Cancel button that gets the toplevel window and destroys it to quit the window.

(g:signal-connect cancelbutton "clicked"
                  (lambda (button)
                    (let ((toplevel (gtk:widget-root button)))
                      (gtk:window-destroy toplevel))))