How can I stop a male feral cat that marks our house by spraying?

You can check this site out for "predator pee" and spray the perimeter of your property:

https://www.predatorpeestore.com/cat-repellent.html

I have used curry powder in the past, it worked for me. You just sprinkle it where the cat is walking on your property, the idea is they do not like it on their paws. No good if it's windy or raining. The other thing was holly leaves or branches with the leaves attached, they do not like prickles on the leaves. These ways will not harm your garden or anyone including animals and birds. The birds will eat the curry powder, I was surprised but they came to no harm. It just changes the colour of their droppings.

There are several options... Bell & Howell makes a solar deterrent, using high pitch sound about $20.00. There's also a device that hooks up to your water hose. Amazon sells these approx. $40.00. Google animal deterrent and you'll see options. My favorite - a big dog! Best of luck!

Can you catch him in a humane trap so you can take him to the vet and be neutered?