At the bottom of each email received from the Googlegroups you will see the following. Simply click on the unsusubscribe link
You received this message because you are subscribed to the Google Groups "A Call To Action" group.
To post to this group, send an email to firstname.lastname@example.org.
To unsubscribe from this group, send email to email@example.com.
Visit this group at http://groups.google.com/group/acalltoaction?hl=en-GB.
For more options, visit https://groups.google.com/groups/opt_out.