Have you setup WPA2 Personal or 128-bit WEP on the Router? I would also recommend disabling AP advertising.
Newer WiFi devices allow you to set the frequencies you may want to use the higher frequencies which don't travel as far and are less likely to be over run by others nearby.
Of course monitor who is connecting use a WiFi analyzer like '''[https://itunes.apple.com/us/app/wifi-explorer/id494803304?mt=12|WiFi Explorer]'''.