Is it coming in when you are not there? Next door's cat will come in at any opportunity. All you can do is deter it with a water pistol, don't leave food lying so it has no attractive reason for coming in. Do your cats try to chase it off? A good cat fight is a great deterrent, my Rover is frequently seen in a furry ball with next door's cat, rolling up and down the garden, it does keep NDC away for a while. perhaps you could try and catch it and see if it is wearing any ID, or ask your neighbours? - at least if you know it is a pet cat you can make sure it's had its flea treatment etc. Short of that, get one of those cat flaps which responds to your cats' microchips. Please don't get collars with magnets, my Rover nearly hung himself on one of those...