Ok im not 100% sure if you have your answer yet or not, seeing as how this thread is years old, but here is what i found to solve this problem. first place all of your computers that you want to have blocked from other wifi networks into an OU. Create a new GPO and link it to the new OU. Go to Computer Configuration>Policies>Windows Settings>Security Settings>Wireless Network (IEEE 802.11) Policies. Create a new Policy (you are give 2 options, xp and vista and later) under the new policies properties, add a policy name and discription then click the Network Permissions Tab. From here you can add the networks that you do not want to deny them access to.
I found that if you add the network that you want to give them access to it requires that you put the security key back in on the computer, So i just deny everything except what i want to leave. You can also prevent the machines from being able to connect to ad-Hocs from here as well.
If you are wanting to block all unknown networks you will have to do the opposite and add the network you want and set it to allow, then tick the box to prevent connections to infrastructure. You will have to add a startup script to add the key backinto the machines. here is a great article on how to do this
Thoughts of a Primary School Tech: Sending out Wireless Settings via GPO with Key
i hope this helps.
