My Own Private War

It is with great sadness that I wish to declare that sides have been established and boundaries declared in my own private war.

For those avid readers of my blog you will recall that earlier this year I rhapsodized about my lovely space in the back of the house. The joy I take in my curly willow tree, my succulents, and the tomatoes I am now eating. I also planted a lovely Japanese lace leaf maple.  Not two days later I noticed that some critter was using the fresh earth as their litter box. I did not want to think that this could be my neighbors' tabby striped cat, Fin. It is only recently that Fin has been allowed out of his apartment. When I drive home he bounds across the road to my car, exactly like a dog would do, to greet me. Soon his forays moved from across the road to parts unknown. He even tried to insinuate himself into my house, but unfortunately I am allergic to cats and Fin could not come in. A territorial war then began between Fin, Hot Toddy, and a feral cat named Bella.  All suspect for using my maple leaf tree as a litter box.
In the meantime, unsure of who the offender was, I went back to the nursery and spent $30 on something the man there assured me was infallible for deterring critters. "Critter Ridder." I followed the directions sprinkling it around - the instructions assured me it would be good for at least 30 days.  Maybe two hours later I noticed that the area was again being used as a toilet.
I then went online, and then sprinkled coffee grounds around, along with the critter ridder. I put down strips of aluminum foil, I placed gravel in the spot,  along with mulch. Nothing worked. After careful observations of the habits of the surrounding cats, I determined that Fin is the culprit. Impartial observers confirmed my suspicion. The other morning I stood in my kitchen and saw Fin sneaking around the house. I grabbed a spray bottle and confronted him. All he did was stare at the bottle, he did not budge. In fact the look he gave me was like, "bring it on, I love this."
And so, Fin uses the space and I clean it out. I built up what I thought to be an impermeable fortress. It wasn't. The other morning I was outside watering my plants and caught Fin, mid act. I picked up a branch of the fuchsia and brandished it in his direction, scaring both myself, and, at last, Fin, who scampered off. Then I saw him crouching beside the house, waiting. I brandished it again.. Off went Fin - for the time being.
Does anyone have any helpful ideas, I would welcome. This war is wearisome!