You can keep a skunk out of your yard by putting the urine of a large predator at the perimeter - wolf urine can often be bought at garden/feed stores.
We had skunks under our house and found that putting mothballs around the opening where they were going in kept them away. Of course, you have to put the mothballs down whilst the skunks are out or you'll trap them in under the house! Mothballs will melt after a while, so you'll need to renew them periodically.
Canine or other predator urine. Although skunks do have there own defense, they prefere to avoid wolves fox and other predators altogether. I believe it is sold at gardening and feed stores in liquid and dry form(just add water). Having a male dog mark up the place works too.