Add Compost to outdoor litter box

If the neighborhood cats are using your gardens as a litter box, you don´t necessarily need to worry about your plants if it is solid waste. Just add some vegetable kitchen scraps to the ´litter box´ area and compost it all into the soil. The composting prevents the strays in my neighborhood from using that area again.
If the area smells of urine, then you will need to flush it with water frequently to dilute the ammonia in the urine.
You will find more tips on preventing the Urine problem in Answers to Common Problems: Spraying/marking Territory.