Been there done that. Moth balls won't work unless the temp under the porch is VERY high, and even then it seldom works, so forget that idea.
Here is what worked for me. Seal around the porch excepr for one (and only one) outlet big enough for the skunk to use as an exit. That doesn't have to be very big. Take a tank vacuum - a shop vac works best - and put the hose on so that it blows outward. Stick the hose through the seal in a spot as far from the opening you left as possible. Heat a gallon or two (how much depends on the space under the porch) of ammonia. It doesn't have to be hot, but if vaporizes better if it is. I heated mine on an outdoor gas grill. Pour the heated ammonia in the vacuum, turn it on and get the hell outa there! If possible, watch from inside to see when it runs out the opening.