TrilbyWill said:
Let's say you have a neighbour who's a great neighbour. Not quite Ned-Flanders level, but in that area.
But let's say you find out they're using your WiFi. If you stop them, they won't stop being nice.
What do you do?
Best course of action in my opinion would be properly securing the network:
WPA2, and a PSK with at least 10 characters (preferably including numbers, upper-case and lower-case letters. Umlauts and special characters don't work with all devices), or EAP, if you have the equipment.
MAC address filtering (I know it can be "cracked" with relatively little effort, but it's like putting one more lock on your door. It might be an easy lock, but it's still more work).
You can also disable DHCP on your router and assign fixed addresses to permitted devices. Again, not an insurmountable obstacle, but one more.
Then you just wait for their reaction, if there is any. Maybe you can come to an "official" arrangement on sharing the network (although that might violate your provider's contract)