To revoke the Marketing Center's access to your Gmail address book:

1. Click on your name in the upper-right corner of any page. In the resulting menu, click on App Authorization:
   
   
   
   
2. Click on Revoke Access on the row with the Gmail logo/icon:
   
   
   
   
3. See the green message box with the phrase "Success! Access to Google Contacts revoked" and the "Authorize" button will be displayed again: