It probably is coming from your own router http://www.btwifi.com/help/about-bt-fon.jsp
When you join BT Fon (or whatever it is now called) then part of that agreement is to enable your BT router to give out the signal to allow other BT customers to use this as a connection.
If you do not want to connect to BT Fon on your own router then you could click off the Connect Automatically tick box on the wireless connection / internet connection ... usually showing on the bottom right of the monitor on a desktop computer.
Hope this helps
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