If you are connected only by the cable then see if you can switch off the wireless bit of your router. That way no-one can evesdrop.
Alternatively, if you still use your laptop wirelessly, make sure that encryption is set on your router. This will prevent all but the determined person accessing your network.
Finally, for maximum security, see if you can add MAC address access control on your router. This will only let those wireless computers to access it if their MAC address (network adaptor hardware code) matches your list.