We have a couple of one-year old cats. A neighbor's slightly older male cat keeps coming to our garden and viciously attacking our cats (usually takes one of them by surprise, corners it and hits it until a person or the other cat distracts him). He is otherwise not violent, is even friendly to us (humans of all ages) and stays away from our cats either if we are in proximity. All three (our cats and the aggressor) are neutered.
As our cats are only out when we're around, we were able to intervene about every time (spray water, make loud noises, run towards them). But this is going on for months now and the intensity of the attacks did not reduce.
How can we best help our cats to improve their interaction with their neighbor (or at least turn it into passive ignorance)?