The FUnjoinOptions parameter specifies the unjoin options. If this parameter is NETSETUP_ACCT_DELETE, the account is disabled ...

The FUnjoinOptions parameter specifies the unjoin options. If this parameter is NETSETUP_ACCT_DELETE, the account is disabled when the unjoin occurs. Note that this option does not delete the account. Currently, there are no other unjoin options defined.  

2 bit - Acct Delete - Delete the account when a domain is left. 
The method returns a 0 if no options are involved.