I've been in exactly the same situation before. I actually used the unsecured connection to print to one of the computers in the house in question. I then knocked on a few doors and found the correct house, informed them of their vulnerability, showed them the printout, then offered to secure it all for them. They were horrified, but very grateful at the same time that i'd taken the trouble to help them out as they had no idea about any of this or how to secure it. They had family pics and all sorts that i could have stolen, then i could have easily locked them out as even their router admin page was unsecure.
I didn't charge them (before anyone asks), i just didn't want to see them become victims of unscrupulous cyber scum.
If ANY of your neighbours would consider you a busybody or whatnot for trying to help, then they can't be very good neighbours can they?!