If you're the administrator, you simply need to go to the Usermap area of JFBConnect. From there, find the user, check the box and click "Delete mapping".
For the end-user to be able to 'break' the link, we actually don't have that option right now. There used to be a method to do so, but it was removed mid-last year due to some complications. We're planning to offer a new way, from the Joomla Profile Editor on the front-end, for the user to break the connection to each social network, if they want.
I hope that helps,
Alex