Currently if user is linked with social service there is no way to de-link it. We are putting this up in the AppHQ release coming next week (before 5th March). We will also add this in API too.
If this is something that is roadblock for you and you can not proceed without it, you have to drop an email to delink it from backend. You can send it to me at email@example.com mentioning your appname, api key and users that you want to delink.