One of our neighbours cats (Oscar) is constantly trying to get into our house.
Generally he can't get in, we have a sureflap for our two, but he's managed it on a couple of occasions recently, once when the sureflap hadn't closed properly and once when we left the back door open. He tends to wander upstairs and go to sleep on a bed.
One of our cats (Horace) obviously takes great exception to this, and if he sees Oscar they fight. If he see Oscar investigating the catflap Horace will whack the catflap to try to stop Oscar getting in (although he can't, generally), and if we let him out the back door he'll actually attack Oscar outside in the garden. Horace is normally quite a happy, chilled cat but increasingly he's sitting on the lookout, monitoring the catflap or looking out the window, and he seems upset because he's not eating as well as he was, or looking for fuss/knees as much.
The problem is none of this puts Oscar off. He just keeps trying the catflap, obviously remembering the time he got in. I squirt him with a water pistol through the catflap, and then go outside and squirt him until he leaves the garden. But still he comes back. He doesn't seem to care about his fights with Horace, although he's the smaller cat. I've actually seen him out on the street and he'll approach dogs, so he's obviously not scared of much. We've done nothing to encourage him. He looks well fed and looked after so his only motive for getting in that I can see is that our house is quiet - the neighbours is full of young children, or perhaps to scrounge any leftover food.
Any suggestions? I don't want to cat proof the garden really because ours like to go in and out. I just don't know what to do to put him off. Its just this particular cat the Horace hates - there are one or two others that I've seen around the place and Horace doesn't seem nearly so bothered about them. They are both neutered males. We have the added problem now that its getting warmer that we can't leave the back door open!