I had a severe problem with cats getting into my rose gardens and other flower beds throughout my property.
You gotta fight a natural instinct with a natural instinct. Do a google search for wolf or coyote urine.
Buy some, you wont need alot. Takin a regular 1 litre spray bottle and mix it 1 part of wolf/coyote urin to 4 parts water. Now there are a few ways you can apply this solution, you can either spray the plant directly, which will obviously achieve the best results, but what worked for me was spraying the surrounding area with the solution, although i made it an even 50/50 solution. Wolf/Coyote urine has a much more pungent smell to it than regular domesticated dog urine.
this worked 100% in my gardens and i saw immeadiate results. After awhile of this, the habit will form with the cat or cats and they will completely avoid the area altogether whether it smells like urine or not. I would start by spraying a few of the walls, floor, and other surrounding areas. Spray a good 15ft radius around the grow area. Only spray your plants with the solution as a last ditch effort, and if you should need to, up the solution to 3 part urine 1 part water. Also, spray the dirt area, not the plant itself. I have 5 cats, i have used this method to train cats to stay away from certain areas, and it is a flawless technique. Be patient with it, but most of the time you will see your cats get curious and then run away. it's kinda funny...
Should they start scent marking more, increase the spray radius of your area. Keep this up until the habit is broken, remember you are trying to break a natural instinctual habit for this animal, its not gonna come easy!
Nova