As I understand things, this is exactly how it's supposed to work with BT. You can share the wifi network of other subscribers, they can share yours in return.
You sign up to say you are happy with this arrangement. I did.
I've never noticed anyone using my wifi, but I live at the end of a private cul de sac so anyone in range of mine would have their own connection anyway because there is no passing traffic
If you didn't sign up to accept the arrangement, summat's not right.