Years ago, I deterred an interloping cat that was bullying our previous kittens by setting the catflap to in-only, trapping it in a laundry basket, and giving it a very thorough soaking with a hose pipe. It avoided our garden for years afterwards. I would hesitate to do that on a very cold day though, especially with a long haired intruder.
My parents lodger (in Tooting Bec) once went a step further and trapped an invading cat in a duvet, walked up to Tooting Common and tossed it (just the cat, not the duvet) into the pond. It climbed out and sprinted off towards Streatham, directly away from its home next door. It took a couple of days to find its way back, during which there was some public "No, I haven't seen your cat" and private ohshitoshitwhathaveidone from Mark. A step too far.
your parents' lodger sounds like a c*nt