What to use to stop cats from marking their territory on the porch?

Use Nature's Miracle No More Marking Pet Stain & Odor Remover in a Spray bottle. You could also leave orange peels around near your front door in the bushes.

Joan, I am having the same problem with wild cats stealing our dog's food and making themselves home....We learnt never to make food available, therefore now we close garage doors and put food away....Can you call any Town/County for help?...They suggested to get some stuff to spray around...JMO

Chances are they are not neutered. They only do what nature tells them to do. Not their fault. See if this helps.

https://www.cuteness.com/article/keep-stray-cat-spraying-house

http://www.humanesociety.org/animals/cats/tips/marking_territory.html

You can buy natural orange spray, cats don't like citrus and loath eucalyptus also. Make your own eucalyptus spray with essential oil in a water spray bottle, shake before using and spraying.

I realize this sounds crazy af, but desperate times call for desperate measures... Check out your local sports/hunting shop for bear or coyote urine. We had a horrible feral cat problem, to the point where our house had fleas (we have no pets) and our kids couldn't play outside because of all the cat crap. They poop and spray everywhere cuz there's no natural predators here. Well, now they think there is lol I sprayed it around the perimeter of the property. Buh bye cats