It's modem/built-in wireless router? In the settings on your "router", the GUI web page in a browser, if you type in whatever it's IP address is, say 192.168.0.1 or 192.168.1.1, which most are normally configured with, you should be at least able to turn off SSID broadcasting so no one can see the signal unless they already knew it was there and connect too it OR possibly turn off wifi entirely. I don't know about the second option, but SSID broadcasting should be attainable to turn off and that would probably take care of it, or just enable filters to block anything except which is already connected currently or that you know will be connected at some point in the future.