Doesn't the law say you can prune any neighbouring plant which overhangs your garden space as long as you give back the prunings to the owner?
I don't see why you can't spray a menace which is coming over and invading and ruining your garden, especially if you happen to need to spray weeds in your won and his just get in the way because they're invading. Do it when he's out, although with all those windows covered in Boston creeper I don't see how he could see you at it. He might however take his revenge later when you do get some good plants of your own in the garden so..........
Take the advice to consult the local environment officer about waste and rubbish and rats too and they may just serve an order on him. If he doesn't clear it up, they will and they'll send him the bill.
And then spray if you have to.