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Hi. Welcome to Just Answer. My name is***** and I've been a veterinarian for 15 years. I'm sorry to hear about this situation with the neighbor cat. If that intact neighbor cat is doing this, most likely he is marking a territory for himself. The best solution would be if the neighbor kept that cat inside, but this is something that is most likely not an option. The next best option is to use one of the ultrasonic deterrents in the area where he's urinating. These can be placed in the areas of the problem. They detect any motion and create a sound that is supposed to be not liked by cats and to hopefully encourage them to not be present in that area. Here is just one example.
With it being outside and your cats being inside, they shouldn't be bothered by it. The insulation of the house should protect them from being spooked by it. Also aiming it out / away from the house will help with this.