Veja como desativar redes ad hoc no Vista. Também deve funcionar no W7
Windows Vista won’t automatically create ad hoc wireless connections with other computers, but for greater security you may want to disable ad-hoc wireless connections entirely. You can do this by running the following Netsh command at an elevated command prompt:
netsh wlan add filter permission=denyall networktype=adhoc
To re-enable ad hoc wireless connections, run this command:
netsh wlan del filter permission=denyall networktype=adhoc