A solution of dish soap and water makes an excellent moss killer. Just take 4 ounces of organic dish soap, add it to a gallon of water, mix them up (but not too vigorously, it shouldn't get foamy) and use a sprayer to apply the mixture to the offending moss.
The dish soap method should begin killing the moss in as little as 24 hours. Another approach is to apply baking soda to the moss for a few days and then rake it out or otherwise remove the moss. For lawns with moss problems, ground limestone will make the soil hostile to moss and friendly to grass.