You could try going next door, kicking their door in, planting one on the teenage suspect, trashing the place a bit on the way out and threatening him with more of the same if he ever uses your wireless network again.
If it stops immediately you'll know it was him. If it doesn't you'll know it's your PC doing an update or something.
Either way you can put your mind at rest, and you won't need to faff around and be clever with IP addresses. You can't lose!