I've also heard citronella oil works (outside!), but avoid using it on cement or similar surfaces as it will stain.
A PELLET GUN is the most effective way to keep stray cats off of your property. A good shot to the rear will discourage him from spraying around your property.
Lurk by an open window and squirt him with a strong jet from a water pistol. Not harmful but the surprise.shock will deter him from coming back as he will then associate your porch with the surprise. You may need to do it a couple of times.
Call you local rescue groups and ask them to pick the poor cats up. They are in need of medical care then they will be put up for adoption.
It is inhumane to force a cat to go outdoors, these kitties will be better off in a loving family.
I've never known a cat that likes mothballs, and only one that can tolerate the smell of vinegar. You might consider spraying the porch and surrounds with undiluted vinegar. You'll have to redo every day or two for a while (or if it rains), but it's better than the smell of cat urine!
If this doesn't work, you may want to put mothballs by each corner of your porch. Good luck!
Do the pet population a favor and trap him with a cruelty -free trap. Sometimes the local humane society will loan these out. Then get him neutered. There are feral cat rescues that can help with costs, but neutering is fairly cheap anyway.
Place white pepper powder in those areas. Cats always sniff and scratch before going to the bathroom. I have the same problem and it really works.
Also it won't hurt anyone, it will just annoy the cats until they find other places to go.
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