You need to go into your router's settings pages and find the Connected Devices or similar setting, then prevent access to any other MAC addresses than those already permitted.
To find your router, press your Windows key and R together and type into the box, including the spaces,
cmd /k ipconfig /all
then press Enter. Note the Default Gateway - something like 192.168.1.1 - and type that into the address bar of Internet Explorer. Login and password will be in your router's manual - post back if you can't get in.