Im not sure there's a spray that can be used. The only way I knowto stop it is to keep the litter box really clean and maybe put newspaper back there or something that makes an unpleasant noise, if you can't stop them.
I'm not sure of a spray but you do need to get rid of the odor from behind there. now when I say this I don't mean that your not cleaning.......what I'm saying is cats can smell spots where they have pottied,as with any animal,but cats have a tendency to keep going in the place they smell....... maybe clean with a soloution of vinager and water,then sprinkle baking soda behind there. the ammonia in the urine is what the kittens smell. I would also put paper behind there incase,they keep going...but you really should keep the litter box clean,with kittens it has to get dirty fast.....cats will much assume go potty somewhere else if they aren't satisfied with the odor of the litter box.when you change the litter wash out the box also.good luck.
There is a cat deterrant that keeps them from going near a certain spot. It's not specifically for pottying. It's not safe on the skin. If it's somewhere you aren't going to be walking (and it sounds that way), you can get that. You can find it at PetSmart. You can also buy some sand called Cat Attract http://www.preciouscat.com/ that really works. My cats waited like half an hour in and out when I first put that stuff in the box. And there was a line at the sandbox.
I'm not sure if there is a spray. I do know you need to get rid of the smell. Oh, you can by this deterrant that keeps cats from going in a certain spot. Here is a good link; http://kitkatzcattery.nstemp.com/litter%20training%for%20kittens.htm Here is another link; www.mismatch.co.uk/catlitter.htm You caould also try putting aluminum foil down. Don't use amonia, they have that in their urine.