Do you expect that your neighbours are stealing your WiFi?  Sure, you could turn on encryption and other security measures, or you could have some fun with them!  Upside-Down-Ternet is a geeky solution to this problem that will show you how to intercept any images that might be sent to your neighbours computer and flip them upside down!  You can be the judge of whether this is too much of an inconvenience to impose upon your neighbors, but it doesn’t do any permanent damage and after all they are stealing from you without giving you the courtesy of asking if you might not mind that they piggy back on your WiFi!