You might want to hire a behaviorist to work with you and your dogs on this issue. Your dogs obviously don't understand that cats in your home are part of their pack and aren't to be touched.
Your dogs likely do not understand where they stand in the pack order of your home. In their minds, they think they're doing a good thing and protecting their territory from interlopers. If they live primarily outside with little human leadership, they will never understand that humans are the top dogs. Dogs have a genetic prey drive, but it can be controlled with time and effort. I'm not talking about screaming at them or beating them when they make a kill. You want to stop the behavior before it happens, and I think you'll need professional help in this area.
What breed of dogs do you have? I would be very concerned about any dog that would kill another animal.