We have a cats coming into our yard and pooping in our plant bed. It's getting pretty gross and the neighbors aren't going to do anything about it. I've read that sprays containing citrus essential oils can work as a deterrent. I was considering making a mixture of water and lemon or citronella oil and spraying it on the ground of the plant bed. Should I be concerned about negative effects on the plants? Maybe making the soil too acidic?
Citronella plants are toxic to cats and dogs, although their reaction depends on the type of exposure and how long for. Most cats do not like the scent of the plant and will avoid it, though dogs are less selective about it. There are some reports of citronella oil poisoning in cats, but not much testing has been done, see here https://www.canadianveterinarians.net/documents/cats-and-essential-oils, but I wouldn't recommend using the dilute oil as a spray, given cats' habit of licking their paws.
Cats don't like citrus peels, though you may not like the appearance of lemon and orange peels strewn across your flower beds. The most effective are branches cut from prickly plants laid across the ground (Berberis, holly and the like) where you know they visit, but again, it's not particularly attractive. Inserting lots of short sticks vertically will deter them too. Whatever you use, you must remove any deposits they have previously left behind first I'm afraid.
Cats like loose soil, so if you have recently dug over an area, that's where they will choose to go; as the soil settles back down and becomes firmer, they will be much less likely to use it, so using short sticks or cut prickly branches till the soil does settle should work. I wouldn't recommend gravel or pea shingle, they tend to view that as cat litter because it's easy to scrape around, but larger pebbles do keep them off. Un-neutered tom cats are less fussy about whether they can scrape a surface up, often just depositing on top or even in un-mown grass.
There are so called cat repellant sprays you can buy, but I never found they worked, and if they did, not for long. They seem not to like mothballs, the old fashioned ones which contain napthalene if you can still find them, but they will need replacing if it rains or after a week or two as the scent fades.
The most effective method of keeping cats off is a sprinkler attached to a movement detector; cats don't like getting wet, so if every time they come along a spray of water gets on them, they will stop coming. Either that or a pebble mulch are the most effective methods.