Two of the guys I ride with regularly carry very loud whistles. They work great. Another simply yells "NO" at the top of his lungs. Most of the time this works too. Pepper spray takes too long to use and is hard to get aimed, etc. when you're at speed. The risk that you might spray yourself or someone else you are riding with also seems to be great. A Glock 9mm also works, but carries similar risks.