Antifungal medications work fine, but not overnight. You have to keep using them consistently until the problem is gone. On the bottle of an athlete's foot powder I've used, it recommends applying the stuff everyday for 4 weeks. One would hope jock itch clears up faster, though.
It's equally important to prevent it from occurring again. Fungal overgrowths that cause problems like jock itch, athlete's foot, yeast infections, diaper rash, etc.