Piggybacking on another persons internet service?

[Marina]

Is there a way to detect this and if so how do you block them?

[Martin]

In your router there should be a page detailing connected devices.
There should also be a page allowing you to block connections and to allow access by MAC address.
You don't give the make / model of your router but best thing is go to the router manufacturers website and get a manual from their product support section. you may even be able to get your internet service provider to connect remotely and assist you.

[cajundude1]

In my case you can't connect unless you have the pass word. See if you router allows the use of a password.