Who actually owns the house? A person/family? A bank? The owner is responsible, and if it's been repo'd the banks usually employ people to mow. If the owner abandoned it, and the bank hasn't taken it yet, it's still legally the owner's problem. And really, someone could make a case for trespassing if you mowed without permission. Silly as that may sound, you should probably just report it to code enforcement and they will contact the legal owner. Sorry for your pain, we lived with that forever until the bank took the neighbor's house.