I understand what you are syaing. I am not sure your neighbours will take kindly to you changing the password without asking first, this is how it should go, act like you know very little about the intarweb: just tell them the next time you see them, don't make a special trip over.
You: Hey, hows it going?
Them: Good and yourself?
You: I am fine, I think there is something wrong with your wireless router...
Them: Oh really, why do you say that?
You: Well I was using my laptop and I can completely access your internet for free and I don't want your other neighbours to start doing this as well...
Them: Is there a way I can stop this from happening?
You: If you want I can put a password on it and then tell you the password.
Them: Okay, if you know how.
You: Oh ya, I will put it on later on tonight, what do you want to set the password to?
Them: CACTUS!
You: Okay, if for some reason your internet asks you for a password it will be cactus.
Them: Okay great, thanks a lot!
You: No problem...
Continue friendly neighbour talk... My own mediocre attempt at humor has been highly amusing to myself.