It displays its main settings on start. You need to select one or multiple of the available blocking options -- disable devices, block with DNS or block with Windows Firewall -- and hit the apply setting button afterwards to block net connections using the selected method(s).
http://www.sordum.org/9660/net-disabler-v1-0/
http://www.softpedia.com/get/System/System-Miscellaneous/Net-Disabler.shtml
http://www.sordum.org/9660/net-disabler-v1-0/
http://www.softpedia.com/get/System/System-Miscellaneous/Net-Disabler.shtml