By default, CircleBack will not sync with your iOS address book. If you have enabled sync and wish to disable it, do the following:
1. Navigate to your native iPhone Settings
2. Select General > Profiles > CircleBack Sync
3. Choose Delete Profile
If you do not see a CircleBack profile listed, you can go into your iPhone settings, press "Mail, Contacts, Calendar" and delete the CircleBack account listed there.
Once you do that, your phone will no longer be synced to CircleBack.