My older cat sprays even though he's been neutered since he was a kitten. He used to only do it when stressed, so I would find the source and fix it. Also I used the pheromone diffusers constantly and they really helped. That is, until they didn't. This last move just pushed him over the edge I think, and he sprays non-stop so I finally put him on Prozac. As long as he takes his meds he is a good, sweet, non-spraying kitty. If he misses a few does he starts spraying again, so I think this is going to be a lifelong thing :(
Since you obviously can't eliminate the source of stress and pheromone diffusers aren't helping, I'd ask about trying Prozac or something similar.