I’m on your side, ok? Your neighbor sounds like a PITA nuisance. Give me a second here, I’ll show you one way to cover yourself. Salt is not likely to kill the tree, the roots will still be there, even if the upper parts were dead, might take years for upper parts to die in any case. Salt WILL fuck up your soil, if it’s sandy soil,the salt could spread a long way beyond your intention and fuck up your groundwater. Enough salt to kill one thing will kill everything, grass, flowers on your side, maybe for years. Contact code enforcement for your city first. Contact a lawyer and ask about your options, a quick consultation doesn’t cost much, you don’t have to retain him. THAT WAY, you look like the reasonable but understandably frustrated GOOD NEIGHBOR to everyone else. Get other people on your side, first.