I know that for a lot of people network security is an issue. This article touches on ways you can determine who is piggy backing on your WiFi network.
Do I Have a Freeloader on my WiFi? - PCWorld
If you find that you do have "extra" wireless clients you should probably look into setting up some security on you router. It is not as complicated as you might think. Your router's website most likely has an easy to follow set of instructions or even a video tutorial to lead you step by step through the procedure.