I tried to clean up smell of cats on my porch with bleach and cayenne pepper, the only thing I accomplished was ruining the carpet on my porch as the cats come back at night and leave cat fur all over the porch. Need other resolution.
About the only thing you can do is get a dog--that should do it. You could set up a sprinker system that is motion activated. Once on the porch, their motion could set off a sprinkler that would soak them. That really would discourage them.
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