This begs the question: If you didn't want anyone on your network, why didn't you lock it down in the first place? In essence, you were broadcasting a signal to the guys home that said "open network right here, please connect!". I always look at open networks as.... well, open for anyone to use. If the owner of the AP doesn't want outsiders to connect to his network, he would lock it down. Right?
Hope you don't do that in the UK, wouldn't matter if the connection was open or not you'd be breaking the law.
NeverNude