If I was your neighbor and you did not secure your wifi connection I would tap in too, and deny it. The only thing you can do is secure your wifi. To do this, log in to your router { get the instruction manual and open an internet window - find out what your router ip is ie:
http://192.168.0.1 ? - The default login is admin with no password probably, look in your instruction manual if not. Look for a security panel or WEP panel. Make a network ID using wep with a 64 bit encryption (128 is not needed for residential use ) and set your network id to a 10 digit number ie: 1234567891 }
To access your wifi connection someone would have to know this network id number, thus securing it.
Good luck!
PS: I was having a bad day a couple weeks ago, so I logged into my a-hole neighbors wifi router and changed his settings so that he could not access anything including his own internet connection. I love setting MAC address restrictions.... Made me feel better - I did change it back after a couple days