Sprinkle some cumin or red pepper spice on the top of the soil - that usually discourages pets. Once you break your cat of the habit you can see what happens without the spice. It won't hurt the ficus and it won't hurt the cat (except for maybe a sneeze).
Put some chicken wire around the pot. Be sure the tips aren't sharp, as you don't want your cat to be pierced by it. You just want to add an obstacle between the cat and the ficus soil.
Place small decorative rocks covering the soil surface. They may not be as enticing to your cat - and they'll help retain moisture on the soil surface.
Make sure your kitty litter box stays clean and enticing - that's where your cat needs to pee!