Look up the Kimber PepperBlaster II.

I have been pushing these lately. I carry one and have had it at the ready for a dog, but never used. It is at least something to have to improve your position. They are fairly light and in a good, no leaking form and not prone to bursting or venting when in your jersey pocket on a hot day. It is a ballistic charge as well, so you are not going to get overspray on yourself. If someone picked up my bike to throw it, they would get it in the face for sure.
http://www.backcountry.com/kimber-pe...9-001b2166c2c0