From Blog:: Craig Pringle - Hide ad-hoc wireless networks in Vista. To remove the possibility of connecting to an ad-hoc network in Vista:

you can, via the command line, filter the list of available wireless networks. To filter out ad-hoc networks run the following from a command line (running as admin, all on one line)

netsh wlan add filter permission=denyall networktype=adhoc

to reverse this again run the following:

netsh wlan del filter permission=denyall networktype=adhoc

Joe