Hiya, I have known a few un-neutered cats who have been lovely cuddly perfect gents- he just sounds like a nightmare!! Would probably still be a nightmare if he was neutered by the sound of it. Some cats, just like some people, are just wa***rs! Try and track down his owners and have a word. Get a spray bottle/ water pistol and fill it with water to spray at it, they hate the sound of the spray and the water. Squeeze a bit of orange/lemon juice in it if you really want to piss the cat off, they generally HATE citrus. Keep chasing it away, maybe eventually it will get the hint. Maybe board up your cat flap for a while and just let your pets in and out the door yourself, then the evil cat may give up trying. I wouldn't suggest anyone taking it upon themselves to neuter someone else's pet, apart from it being very presumptuous, you could be asking for a lawsuit!