If you meant to unsubscribe -
Try sending a blank e-mail to
security-dev...@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 -
2. Click on the button that has an icon of a head and shoulders.
3. Click on "Leave group" and you are done. Or -
a. Select in the selection box - "Don't send email updates".
b. Click on "Save".