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unless you can lock them out of configuring these devices, all they have to do is connect to the t'internet some other way. And if you did that, then they can only access the the net from with in range of your kit.
A proxy is used to stop your server serving things you don't want it to. All they need to do is go through next door's wifi.

Make them pay for the bandwidth, that'll make a bit more wary.