Jolis Cheveux Seems to do the Trick!
I am a Safe Routes to School bike safety instructor and we use fleets of loaner bikes and helmets at all the schools we visit for kids who don't bring their own bikes or helmets. We've been using Jolis Cheveux Helmet Spray for a few years now and have thus far not encountered any cases of lice or nits in our helmets. We give 2-3 sprays after each helmet use and have been pleased with the results. We also chose this product for its all natural composition and pediatrician-developed formula. It smells pleasant, too. We recommend trying it!