How to keep cats off property legally and humane? Can I purchase a water gun and squirt them with it or is that unethical?

Squirting with water is okay.

Water is perfectly harmless so there's no reason for that to be unethical. When I was younger I had a wonderful cat who was an indoor/outdoor cat. At night, her way in and out was through my bedroom window which was just over the front porch roof. I never got a solid night's sleep because she was always waking me up to let her in or let her out. When she wanted in she would stand up outside the window and literally pound on the glass with her paws. When she wanted out she would sit on the windowsill and pluck at the screen with her claws like a harp until I let her out. I had a tiny little squirt gun that I kept next to the bed, and when she started plucking at the screen I'd slowly reach for it and give her a squirt to chase her off the window. It didn't take long before she would run off every time I started to move for it. She was such a feisty cat. I loved her so much and I miss her a lot.

You could build a wall.

Sure squirt them. Side note: in my area the animal shelters will not take adult cats. Apparently they used to do that but then ended up with a zillion cats and had to euthinize them. For that they were criticized in the media and people got upset about it so the shelters had to stop accepting cats all together. They will, however, accept kittens presumably because they are easy to place.

That's fine. You'll never keep them off forever, it's kind of a waste of time and energy.

with a water pistol it will be fun but add lemon juice or coffee to the water since for them it is an unpleasant smel

pull the caffe Ñarro in the potsl.

lime plant or lemon trees

What on earth is your problem? You can't just abuse animals. What a freak - it's shameful that your parents allowed you to grow up this way.

Set live traps and haul them off to the local shelter.