What can I mix in the soil of my flower box to deter my kittens from using it as a potty?

I have an 12 foot flower box outside of my bedroom windows (it runs across the front wall of my house), I also have 2 6-month old kittens who insist on using it as a litter box. Now the front of my house smells incredibly bad whenever you get near the front door. I have plastic flowers inbedded in the box so theres no living organisms in danger of losing life, but I don't want any harm to come to the kittens. I thought mixing coffee grounds might deter them and help with the smell, but if theres something else as harmless to use I'm open to the idea.