Not sure if this is the correct way but i have done it, as shown below, on my Laptop with Windows 7
1 Start button and select control panel.
2 Selct Network and Internet
3 Select Network and Sharing Centre
4 In right hand column select Manage Wireless Networks.
This should bring up a list of networks.
5 Right click on network you want to remove and select Remove Network
It worked for me..Good luck.
Jean