Always remember, in order to rely on the defenses of self defense, defense of others or defense of property, your actions must be reasonable under the circumstances. I would strongly advise against using pepper spray or any other weapon in this situation. I think video taping or otherwise photographing anyone who comes near you or your car is a great idea and the images and sounds you capture would certainly be admissible in a future action (civil suit, criminal case) against anyone who damages your property or otherwise assaults you.
As far as using physical force against someone, you will never be in a position to do so if you are seated behind the wheel of your car and your assailants are outside the car throwing things, ect. Think about it - any force you might use would have to come after the fact and would be viewed as retaliation. While a person might be legally justified in using force to prevent someone from destroyer her personal property, she may never use force to retaliate against someone who has already damaged or destroyed the property. Again, if you have sufficient proof that a specific person damaged your paint job or dented your car, you can and should go after that person both criminally and civilly.
I agree with Mr. Gordon's advice. I would add something though. What you are thinking about (using pepper spray) would require you either getting out of the car of rolling down the window. Since we are taking as a given that you have already been attacked with thrown coffee cups and signs, WHY WOULD YOU WANT TO DO THAT? Do you have a death wish? Or is punishing them more important than your safety? Try to remember, if you lower the window, you are seated, maybe restrained by your seat belt, and they have freedom of movement to throw punches, objects or hit you with a wrench. Engaging them in counter violence could well be the dumbest thing you've ever done. I strongly recommend against it. Even taking photos could cause escalation, but it's better than engaging in a fight of one against twenty.
Our law office is in Tustin, California and we are familiar with local laws that pertain to self defense of property.
In general you are entitled to use "reasonable force" to protect yourself if you feel that your personal safety is in jeopardy. If the matter goes to court it will be a "reasonable person test". What this means is the jury would be asked whether a reasonable person in your situation would feel their personal safety was being placed at risk, and if so you can use reasonable force to defend yourself.
As to taking videotape, of course that is permissible and you can turn that evidence over to the local police department and they will determine whether a crime has been committed upon you or your property. Hope this helps.
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.