On one hand, I appreciate those who leave their WiFi networks open so anyone in range can access them.
But on the other hand, that opens you up for a rather large can o' legal worms that I, for one, am unwilling to deal with myself.
If you've left it open/unsecured, then you are broadcasting unencrypted information across public over-the-air frequencies.
Anyone can legally receive that information & interact with it as they see fit.
Unless they do something illegal on your network, there's not much you can DO about it.
If you don't want them accessing it, encrypt it, or stop broadcasting altogether. =J