Go to Start > Network > Network and Sharing Center
click Manage network connections under 'Tasks'
Right-click Wireless Network Connection
Select Properties.
In the Wireless Network Connection Properties dialogue box, click the
Wireless Networks tab.
Select other wireless network like "Y5ZONE", "PCCW" and click Properties
Click Connection tab
Uncheck the box "Connect when this network is in range"
This is a copy and paste answer from another internet forum. It sounds correct to me but I have not run through it step by step.