From Facebook Developer Wiki

Description

This method allows a site to unregister a previously registered account (using connect.registerUsers). You should call this method if the user deletes his or her account on your site.

Parameters

Required	Name	Type	Description
required	api_key	string	The application key associated with the calling application. If you specify the API key in your client, you don't need to pass it with every call.
	sig	string	An MD5 hash of the current request and your secret key, as described in the How Facebook Authenticates Your Application. Facebook computes the signature for you automatically.
	v	float	This must be set to 1.0 to use this version of the API. If you specify the version in your client, you don't need to pass it with every call.
	email_hashes	array	An array of email_hashes to unregister.

Response

This method returns an array of unregistered email hashes. If any email hashes are missing, we recommend that you try unregistering the account again later.