bill - 1:45 am on Sep 28, 2011 (gmt 0)
Feedfetcher does not follow robots.txt guidelines.
If your feed is publicly available, Google can't restrict users from accessing it. One solution is to configure your site to serve a 404, 410, or other error status message to user-agent Feedfetcher-Google.
It's understandable that you don't want to lose the subscribers. Maybe adding a large eye-catching image to the Please unsubscribe message might draw more attention.