Tips for Keeping Cats Away

Here is what I've found to deter cats from areas they are unwanted and prevent spraying.
Lime Essential Oil: My cats hate this smell. I splash a little in the area and they avoid it. Continuous use in an area eventually led to them leaving that area alone (e.g. countertops and houseplants).

Canned Air: If you see a cat approaching an unwanted area, use some canned air like you get in an office supply store and use to dust keyboards. All cats hate the hiss sound of this air.

If you can't be around enough, invest in a motion sensor topper for the canned air. It can be expensive, but it will cut down on the amount of cleaning you have to do and soon have the cats avoiding the area.
Enzymatic Cleaners: Lastly, invest in some good enzymatic urine cleaner. If homemade recipes don't work, you need the added power of these cleaners. I buy Bio-kleen from Amazon. Urine Destroyer works on really saturated areas. Cats will keep spraying an area that smells like urine, especially other cats' urine. You have to remove "all" the smell or they keep coming back.