Try sending a blank e-mail to
chromium-app...@chromium.org (note that +unsubscribe in the address, it is important). If you already did exactly that, skip this.
If that does not work and you are still getting e-mails -
1. Go to http://groups.google.com/a/chromium.org/group/chromium-apps/subscribe?hl=en.
2. You might need to log in. If you do -
2a. Click on the "Enter your email address to access the document" link, at the bottom of the "Sign in to your account at Chromium.org" box.
2b. Type your e-mail there and press Enter.
2c. Log in regularly, if it asks you to.
3. Click on "Unsubscribe".