I commented on what I think is the futility of using Liquid Fence to discourage Tabby from visiting, but before I totally evaporate into the woodwork, here’s another thought that probably would work although I have never used it with cats in mind.
Spray the bottom of your door and a portion of the deck floor with plain cheap ammonia. Cats have sensitive noses and that should discourage the furry critter. It keeps the local bear(s) (and racoons) from rooting in our garbage cans so that just might work. You would need to do it each day for a while until Kitty gets the message. Need cheap spray bottle from Home Depot, Lowes, Wallyworld and some generic ammonia (non sudsing). I use it full strength to keep Bruin away. I spray the outside of the cans, and spray each bag I toss in. Worst that happened is that rain washed off enough on the outside that Bruin knocked over the can. Managed to grab a bag and try to run off with it, got less than three feet and dropped his/her booty like a hot potato and ran..
If nothing else, if you have any friends in bear country, let them know about this (also keeps stray dogs away from stuff too..)
No, no no no no NO! Bad idea!
There’s a reason anyone cleaning up cat marking or an accident is told NEVER use ammonia. Urine contains ammonia and this is likely to start a marking war! This is quite literally the worst advice that could be given...I know the person who posted it means well, and the idea of offending their nose isn’t entirely bad, but ammonia is the worst choice of anything.
I tend to hear of citrus being used as a deterrent but I’m not sure if it’s a scent deterrent or a taste one, and obviously only a scent one is relevant. Even then it may not...I’ve seen a cat or two that turns out to LOVE citrus. Still, orange peels/grating is cheap enough, if you buy and eat the orange yourself. Research that if you want.
Someone suggested mothballs, which could work. I don’t know of any contraindications for that, other than the cat might be dumb enough to chow down...hopefully not!
A non chemical deterrent that might work is tin foil. Place some on the area where the cat is getting, taped down enough to not blow away in the wind but loosely enough to crinkle and rattle. Many cats hate this underfoot.
Adding in the interest of full disclosure...I asked hubby if he knew and he swears ammonia will work, but I’ve always heard and seen the opposite.