I know this is a really old question, but I just wanted to share my experience and explain what would really work...
I recently competed in a mixed gender kick-boxing competition where a strike to the balls was allowed and guys were not allowed to wear any protection (protective cups etc) at all. I was competing against a girl and was winning comfortably on points going into the very last round, but with like seconds to go, I lost concentration and ended up facing the girl with my legs wide apart, so she literally ran up and kicked me in the balls as hard as she possibly could to win!
I immediately screamed out in shock, fell to the floor and was just rolling around screaming in absolute agony while the girl celebrated her win! A lot of the girls watching in the audience were laughing and cheering too! What made it worse was that my girlfriend was sat watching on the very front row and of course she was horrified!
I would not wish that pain on anyone and my balls were very bruised and swollen for at least a week! It was excruciatingly agonising! :o(