So sorry to hear you have had to go thru this and you have rec'd excellent advice. I too have wirless at home and noticed that two of my neighbours have wirless that is not even encrypted or password protected. I ended up doing it for them but as someone said already, someone driving through (we tend to have a very caring quiet neighbourhood) could easily have used their wireless account.
Have you attempted to obtain the ISP and IP of the "Joe Blow"? I had one nasty virus attack one of my computers once - and we have virtually (pun intended) every good anti virus, anti spyware, popups,etc software going but this guy - who as it turned out had contracts for companies like Sears - had attached his %^$# to a popup and it was almost impossible to get rid of. I did tho but it was the most difficult one ever! I traced his IP address and emailed him and reported him to the BBB, if that helps but we have poor legislation re these things. He was operating out of west end Mtl as it turned out. But it would not hurt to find this guy because he obviously stole bandwidth and broke into your home, virtually.
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer